Una forma de seguir las actividades CULTURALES y CIENTIFICAS:
Los derechos de autor son respetados, siendo borrados los contenidos al recibir aviso de su mal uso.
La privacidad es respetada escrupulosamente, suprimiéndose cualquier imagen fotográfica que sea cuestionada.
Palo
Alto, en California, alberga la afamada Universidad de Stanford. En
el Shopping de la ciudad se come bien en Creaming. Pido una
hamburguesa con buena carne, acompañada de unas patatas fritas
estupendas que sirven con algún borde del trozo con la cáscara
sin pelar. Éste es el país de las patatas y los
tomates, del chocolate y la vainilla, fina y riquísima,
productos que trajeron a España Colón, Hernán
Cortés y otros descubridores. Pero las guarniciones de todo
tipo de verdura saben a jarabe o amargan. Aqui refinan tanto, que ni
la sal sazona ni el azúcar endulza.
Enfrente
tengo una mesa con una pareja, seguramente novios a juzgar por los
gestos complacientes y tímidos de ella. Como todas las chinas,
es bajita y regordeta, de cara con mofletes y mirada inexpresiva, y
eso que ríe con ganas pero sin gracia. El novio, blanco, es
guapete de cara, con mirada de panoli, de barba cerrada perfectamente
rasurada. Enfrente del pringao, e invisible para mí, está
su madre; luego, cuando salga, comprobaré que tiene cara de
ser una suegra de armas tomar, cuando le toque. A la izquierda de la
suegra se sienta el suegro, cuyo perfil muestra y demuestra de dónde
procede la cara guapa del novio de la china, que podría ser
japonesa, tailandesa, pequinesa, camboyana, taiwanesa, hongkonesa…,
de toda la raza oriental hay muestras en la Bahía, e incluso
vietnamita, que después de darles para el pelo allí a
los de aquí, han venido a cortárselo en las
peluquerías.
Al
levantarse los cuatro, observo que la china camina parsimoniosa como
si tuviera, como todas, los pies planos. Y la incipiente familia se
va.
Mientras
tanto, a la family nuestra le surge un problema: hay que calentar el
potito para la pequeña. Tras un rato largo, que me va poniendo
de los nervios, aparece la camarera después de varias manos
levantadas sin resultado. “Dejarme solo”, les digo a mi
wife (esposa) y a mi daughter (hija). Le pongo a la rubia cuarentona
el potito en la mano. “Cold, please” (no es así,
porque no me entiende, y Colt no es porque es un revólver del
Fart West, Lejano Oeste: eso me lo sé de las pelis de
vaqueros). “Hold, please” (tampoco: nai flowers, ni
flores). “Hot, please” (todavía duda). “¡Que
lo calientes, coño!”. Entonces siento sobre mí la
mirada sorprendida de mi daughter, la mirada nada complaciente de
quien tiene sobre mí mucho ascendiente (my wife) y la mirada
reticente de la mesera (así lo dicen los mexicanos). Su face
(cara) me mira de medio lado, levanta su mano (hand), llena de dedos
llenos de anillos, y la acerca a my face: “Ésta me hace
una cara nueva”, pienso mientras, glub, trago saliva y noto el
calorcillo nada agradable de la mirada furibunda de my wife sobre mi
nuca (nuca no sé decirlo en inglés, como otros cientos
de palabras: a que se me nota).
La
cuarentona nada culona coge el potito, y, altiva cual princesa
ofendida, se va hacia la cocina y mira de soslayo mascullando algo en
inglés, que oigo pero no entiendo. Ahora me siento como Mister
Bean; pero, bueno, he salido indemne, he salvado el pellejo. Una vez
más, he superado, con nota, mi genética facilidad para
ir dando la nota (¿cómo se dirá esta frase en
inglés?). Pues eso tampoco lo sé… ¡Mira
que son difíciles de entender estos yanquis!
People have long asked:
"What is the world made of?" and "What holds it together?" What is the World Made of?
Why do so many things in this world share the same characteristics?
People have come to realize that the matter of the world is
made from a few
fundamental building blocks of nature.
The word "fundamental" is key here. By fundamental building blocks we
mean objects that are
simple and structureless -- not made of anything smaller.
Even in ancient times, people sought to organize the world around them into fundamental
elements, such as earth, air, fire, and water.
Today we know that there is something more fundamental than earth, water, air, and fire...
THE ATOM
By convention there is color, By convention sweetness, By convention bitterness, But in reality there are atoms and space.
-Democritus (c. 400 BCE)
Around 1900, people thought of atoms as
permeable balls with bits of electric charge bouncing around inside.
Atomic theory (Wikipedia)
In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms,
as opposed to the obsolete notion that matter could be divided into any
arbitrarily small quantity. It began as a philosophical concept in
ancient Greece (Democritus) and India and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of particles.
The word "atom" (from the ancient Greek adjective atomos, 'indivisible'[1])
was applied to the basic particle that constituted a chemical element,
because the chemists of the era believed that these were the fundamental particles of matter. However, around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "indivisible atom" was actually a conglomerate of various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other. In fact, in certain extreme environments such as neutron stars,
extreme temperature and pressure prevents atoms from existing at all.
Since atoms were found to be actually divisible, physicists later
invented the term "elementary particles" to describe indivisible particles. The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter.
But is the atom fundamental?
The Standard Model - What is fundamental?
Is the Atom Fundamental?
People soon realized
that they could categorize atoms into groups that shared similar chemical
properties (as in the Periodic Table of the Elements). This indicated
that atoms were made up of simpler building blocks, and that it was these
simpler building blocks in different combinations that determined which
atoms had which chemical properties.
Moreover, experiments which "looked" into an atom using particle probes indicated that atoms
had structure and were not just squishy balls. These experiments helped scientists determine
that atoms have a tiny but dense, positive nucleus and a cloud of negative electrons (e-).
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively chargedelectrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule.
An atom containing an equal number of protons and electrons is
electrically neutral, otherwise it has a positive charge if there are
fewer electrons (electron deficiency) or negative charge if there are more electrons (electron excess). A positively or negatively charged atom is known as an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.[1]
The name atom comes from the Greekἄτομος (atomos, "indivisible") from ἀ- (a-, "not") and τέμνω (temnō, "I cut")[2], which means uncuttable, or indivisible, something that cannot be divided further.[3] The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists
provided a physical basis for this idea by showing that certain
substances could not be further broken down by chemical methods. During
the late 19th and early 20th centuries, physicists
discovered subatomic components and structure inside the atom, thereby
demonstrating that the 'atom' was divisible. The principles of quantum mechanics were used to successfully model the atom.[4][5]
Atoms are minuscule objects with proportionately tiny masses. Atoms
can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.94% of an atom's mass is concentrated in the nucleus,[note 1]
with protons and neutrons having roughly equal mass. Each element has
at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus.[6] Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons
that match the energy differences between the levels. The electrons
determine the chemical properties of an element, and strongly influence
an atom's magnetic properties.
An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud
distribution (black). The nucleus (upper right) in helium-4 is in
reality spherically symmetric and closely resembles the electron cloud,
although for more complicated nuclei this is not always the case. The
black bar is one angstrom (10−10 m or 100 pm).
The periodic table is a tabular display of the chemical elements, organized on a basis of their properties. Elements are presented in increasing atomic number; while rectangular in general outline, gaps are included in the rows or periods to keep elements with similar properties together, such as the halogens and the noble gases, in columns or groups, forming distinct rectangular areas or blocks.[1]
Because the periodic table accurately predicts the properties of
various elements and the relations between properties, its use is
widespread within chemistry, providing a useful framework for analysing
chemical behavior, as well as in other sciences.
Although precursors exist, the current table is generally credited to Dmitri Mendeleev, who developed it in 1869 to illustrate periodic trends in the properties of the then-known elements;[2] the layout has been refined and extended as new elements have been discovered and new theoretical models developed to explain chemical behavior.[3] Mendeleev's presentation also predicted some properties of then-unknown elements
expected to fill gaps in his arrangement; these predictions were proved
right when said elements were discovered and found to have properties
close to the predictions.
All elements from atomic numbers 1 to 118 have been isolated. Of these, all up to plutonium
exist naturally in significant quantities; the rest have only been
artificially synthesised in laboratories, along with numerous synthetic radionuclides
of naturally occurring elements. Production of elements beyond
ununoctium is being pursued, with the question of how the periodic table
may need to be modified to accommodate these elements being a matter of ongoing debate.
The standard layout of the periodic table. The colors represent different categories of elements.
Is the Nucleus Fundamental?
Because it appeared small, solid, and dense, scientists originally thought that the nucleus
was fundamental. Later, they discovered that it was made of protons (p+), which are positively charged,
and neutrons (n), which have no charge.
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The proton–neutron model of nucleus was proposed by Dmitry Ivanenko in 1932.[citation needed] Almost all of the mass of an atom is located in the nucleus, with a very small contribution from the orbiting electrons.
The diameter of the nucleus is in the range of 1.75 fm (femtometre) (1.75×10−15 m) for hydrogen (the diameter of a single proton)[1] to about 15 fm
for the heaviest atoms, such as uranium. These dimensions are much
smaller than the diameter of the atom itself (nucleus + electronic
cloud), by a factor of about 23,000 (uranium) to about 145,000
(hydrogen).
The branch of physics concerned with studying and understanding the
atomic nucleus, including its composition and the forces which bind it
together, is called nuclear physics.
Are protons and neutrons fundamental?
Physicists have discovered that protons and neutrons are composed of even smaller
particles called quarks.
As far as we know, quarks are like points in geometry. They're not made up of anything else.
After extensively testing this theory, scientists now suspect that quarks and the electron
(and a few other things we'll see in a minute) are fundamental.
Protons (Wikipedia)
The proton is a subatomic particle with the symbol p or p+ and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.
In the standard model of particle physics, the proton is a hadron, composed of quarks. Prior to that model becoming a consensus in the physics community, the proton was considered a fundamental particle. A proton is composed of two up quarks and one down quark, and is about 1.6–1.7 fm in diameter.[2]
The free proton is stable and is found naturally in a number of situations. Free protons exist in plasmas
in which temperatures are too high to allow them to combine with
electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay, and also result from the decay of free neutrons, which are unstable. In all such cases, protons must lose sufficient velocity and (kinetic energy)
to allow them to become associated with electrons, since this is a
relatively low-energy interaction. However, in such an association, the
character of the bound proton is not changed, and it remains a proton.
The attraction of low-energy protons to electrons, either free
electrons or electrons as present in normal matter, causes such protons
to soon form chemical bonds with atoms. This happens at sufficiently
"cold" temperatures (comparable to temperatures at the surface of the
Sun). In interaction with normal (non plasma) matter, low-velocity free
protons are attracted to electrons in any atom or molecule with which
they come in contact, causing them to combine. In vacuum, a sufficiently
slow proton may pick up a free electron, becoming a neutral hydrogen
atom, which then will then react chemically with other atoms if they are
available and sufficiently cold.
The quark structure of the proton. (The color assignment
of individual quarks is not important, only that all three colors are
present.)
The neutron is a subatomichadron particle which has the symbol n or n0, no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of protons in a nucleus is the atomic number and defines the type of element the atom forms. Neutrons are necessary within an atomic nucleus as they bind with protons via the strong force; protons are unable to bind with each other due to their mutual electromagnetic repulsion being stronger than the attraction of the strong force. The number of neutrons is the neutron number and determines the isotope of an element. For example, the abundant carbon-12 isotope has 6 protons and 6 neutrons, while the very rare radioactive carbon-14 isotope has 6 protons and 8 neutrons.
While bound neutrons in stable nuclei are stable, free neutrons are unstable; they undergo beta decay with a mean lifetime of just under 15 minutes (881.5±1.5 s).[4] Free neutrons are produced in nuclear fission and fusion. Dedicated neutron sources like research reactors and spallation sources produce free neutrons for use in irradiation and in neutron scattering experiments. Even though it is not a chemical element, the free neutron is sometimes included in tables of nuclides.[5] It is then considered to have an atomic number of zero and a mass number of one, and is sometimes referred to as neutronium.[citation needed]
The neutron has been the key to nuclear power production. After the
neutron was discovered in 1932, it was realized in 1933 that it might
mediate a nuclear chain reaction. In the 1930s, neutrons were used to produce many different types of nuclear transmutations. When nuclear fission
was discovered in 1938, it was soon realized that this might be the
mechanism to produce the neutrons for the chain reaction, if the process
also produced neutrons, and this was proven in 1939, making the path to
nuclear power production evident. These events and findings led
directly to the first man-made nuclear chain reaction which was
self-sustaining (Chicago Pile-1, 1942) and to the first nuclear weapons (1945).
The quark structure of the neutron. (The color assignment of individual quarks is not important, only that all three colors are present.)
The Rutherford model or planetary model is a model of the atom devised by Ernest Rutherford. Rutherford directed the famous Geiger-Marsden experiment in 1909, which suggested on Rutherford's 1911 analysis that the so-called "plum pudding model" of J. J. Thomson of the atom was incorrect. Rutherford's new model[1]
for the atom, based on the experimental results, had the new features
of a relatively high central charge concentrated into a very small
volume in comparison to the rest of the atom and containing the bulk of
the atomic mass (the nucleus of the atom).
Rutherford's model did not make any new headway in explaining the
electron-structure of the atom; in this regard Rutherford merely
mentioned earlier atomic models in which a number of tiny electrons
circled the nucleus like planets around the sun, or a ring around a
planet (such as Saturn). However, by implication, Rutherford's
concentration of most of the atom's mass into a very small core made a
planetary model an even more likely metaphor than before, as such a core
would contain most of the atom's mass, in an analogous way to the Sun containing most of the solar system's mass. Rutherford's model was later corrected by Niels Bohr.
atomic model Rutherford: electrons (green) and nucleus (red).
In atomic physics, the Bohr model, introduced by Niels Bohr in 1913, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model
(1911). Since the Bohr model is a quantum-physics–based modification of
the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.
The model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen.
While the Rydberg formula had been known experimentally, it did not
gain a theoretical underpinning until the Bohr model was introduced. Not
only did the Bohr model explain the reason for the structure of the
Rydberg formula, it also provided a justification for its empirical
results in terms of fundamental physical constants.
The Bohr model is a primitive model of the hydrogen atom. As a theory, it can be derived as a first-order approximation of the hydrogen atom using the broader and much more accurate quantum mechanics, and thus may be considered to be an obsolete scientific theory.
However, because of its simplicity, and its correct results for
selected systems (see below for application), the Bohr model is still
commonly taught to introduce students to quantum mechanics, before
moving on to the more accurate but more complex valence shell atom. A related model was originally proposed by Arthur Erich Haas in 1910, but was rejected. The quantum theory of the period between Planck's discovery of the quantum (1900) and the advent of a full-blown quantum mechanics (1925) is often referred to as the old quantum theory.
The Rutherford–Bohr model of the hydrogen atom (Z = 1) or a hydrogen-like ion (Z > 1), where the negatively charged electron confined to an atomic shell encircles a small, positively charged atomic nucleus and where an electron jump between orbits is accompanied by an emitted or absorbed amount of electromagnetic energy (hν).[1] The orbits in which the electron may travel are shown as grey circles; their radius increases as n2, where n is the principal quantum number. The 3 → 2 transition depicted here produces the first line of the Balmer series, and for hydrogen (Z = 1) it results in a photon of wavelength 656 nm (red light).
Illustration of the vector model of orbital angular momentum.
This is the modern atom model.
Electrons are in constant motion around the nucleus, protons and neutrons jiggle within the nucleus, and quarks
jiggle within the protons and neutrons.
This picture is quite distorted. If we drew the atom to scale and made
protons and neutrons a centimeter in diameter, then
the electrons and quarks would be less than the diameter of a hair and
the entire atom's diameter would be greater than the length of
thirty football fields! 99.999999999999% of an atom's volume is just empty space!
Electrons, Protons And Neutrons | Standard Model Of Particle Physics
While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons
are at least ten thousand times smaller than that. We don't know exactly how small quarks and electrons are;
they are definitely smaller than 10-18 meters, and they might literally be points, but we
do not know.
It is also possible that quarks and
electrons are not fundamental after all, and
will turn out to be made up of other, more
fundamental particles. (Oh, will this
madness ever end?)
What are we looking for?
Physicists constantly look for new particles. When they find them, they
categorize them and try to find patterns that tell us about how the fundamental building
blocks of the universe interact.
We have now discovered about two hundred particles (most of which aren't fundamental). To
keep track of all of these particles, they are named with letters from the Greek and Roman alphabets.
Of course, the names of particles are but a small part of any physical theory. You should
not be discouraged if you have trouble remembering them. Take heart: even the great Enrico Fermi
once said to his student (and future Nobel Laureate) Leon Lederman,
"Young man, if I could remember the names of these
particles, I would have been a botanist!"
The standard model
Physicists have developed a theory called The Standard Model
that explains what the world is and what holds it together.
It is a simple and comprehensive theory that explains
all the hundreds of particles and complex interactions with only:
6 quarks.
6 leptons. The best-known lepton is the electron. We will talk about leptons in just a few pages.
Force carrier particles, like the photon. We will talk about these particles later.
All the known matter particles are composites of quarks and leptons, and they interact
by exchanging force carrier particles.
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong
nuclear interactions, which mediate the dynamics of the known subatomic
particles. Developed throughout the mid to late 20th century, the
current formulation was finalized in the mid 1970s upon experimental
confirmation of the existence of quarks. Since then, discoveries of the bottom quark (1977), the top quark (1995) and the tau neutrino
(2000) have given further credence to the Standard Model. Because of
its success in explaining a wide variety of experimental results, the
Standard Model is sometimes regarded as a theory of almost everything.
Still, the Standard Model falls short of being a complete theory of fundamental interactions because it does not incorporate the physics of dark energy nor of the full theory of gravitation as described by general relativity. The theory does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not correctly account for neutrino oscillations
(and their non-zero masses). Although the Standard Model is
theoretically self-consistent, it has several apparently unnatural
properties giving rise to puzzles like the strong CP problem and the hierarchy problem.
Nevertheless, the Standard Model is important to theoretical and experimental particle physicists alike. For theorists, the Standard Model is a paradigmatic example of a quantum field theory, which exhibits a wide range of physics including spontaneous symmetry breaking, anomalies, non-perturbative behavior, etc. It is used as a basis for building more exotic models which incorporate hypothetical particles, extra dimensions and elaborate symmetries (such as supersymmetry)
in an attempt to explain experimental results at variance with the
Standard Model, such as the existence of dark matter and neutrino
oscillations. In turn, experimenters have incorporated the standard
model into simulators to help search for new physics beyond the Standard Model.
Recently, the standard model has found applications in fields besides particle physics, such as astrophysics, cosmology, and nuclear physics.
As you have read, everything from galaxies to mountains to molecules is made from quarks and leptons. But that is not the whole story. Quarks behave differently than leptons, and for each kind of matter particle there is a corresponding antimatter particle.
Matter and antimatter
For every type of matter particle we've found, there also exists a corresponding antimatter particle, or antiparticle.
Antiparticles look and behave just like their corresponding matter
particles, except they have opposite charges. For instance, a proton is
electrically positive whereas an antiproton is electrically negative.
Gravity affects matter and antimatter the same way because gravity is
not a charged property and a matter particle has the same mass as its
antiparticle.
When a matter particle and antimatter particle meet, they annihilate into pure energy!
bubble chamber photo
Bubble chambers were an important kind of particle detector from 1953 well into the
1970s. The idea behind a bubble chamber is that when you shoot charged particles into
a superheated liquid, the particles will leave behind a track of bubbles.
This makes it easy to track the particles and figure out important things like their
charge and mass.
(A superheated liquid is made by lowering the pressure in the
chamber when the liquid is just below the boiling point.)
The magnetic field
in this chamber
In order to find out things about particles, physicists measure their charge
and momentum. To do this they observe particle collisions in strong magnetic fields,
because different kinds of particles behave very differently in a magnetic field depending on
their charge and their momentum.
For one thing, the signs of charged particles
can easily be read from their paths, since they curve in opposite directions in the same
magnetic field.
For another, the momenta of particles can be
calculated easily because the path of a particle with greater momentum bends less than that
of one with lesser momentum.
Antimatter (Wikipedia)
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example, a positron (the antiparticle of the electron or e+) and an antiproton (p) can form an antihydrogen atom in the same way that an electron and a proton form a "normal matter" hydrogen atom. Furthermore, mixing matter and antimatter can lead to the annihilation of both, in the same way that mixing antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The result of antimatter meeting matter is an explosion.[1]
There is considerable speculation as to why the observable universe
is apparently composed almost entirely of matter (as opposed to a
mixture of matter and antimatter), whether there exist other places that
are almost entirely composed of antimatter instead, and what sorts of technology might be possible if antimatter could be harnessed. At this time, the apparent asymmetry of matter and antimatter in the visible universe is one of the greatest unsolved problems in physics. The process by which this asymmetry between particles and antiparticles developed is called baryogenesis.
Fermilab scientist Don Lincoln describes
antimatter and its properties. He also explains why antimatter, though
a reality, doesn't pose any current threat to our existence!
Public Lecture—ANTIMATTER: What is it and where did it go?
Lecture Date: Tuesday, October 28, 2008.
In this public lecture we will explore the mystery of antimatter: Where
did it go? Why is the universe made up of only matter, with no
observable antimatter? And why does the universe have any matter left in
it anyway? The SLAC "B"-Factory was built to answer these questions.
Over the last decade, almost a billion "B"-mesons were created and
studied at the B-Factory to search for subtle differences between matter
and antimatter, differences that lie at the heart of the antimatter
mystery. We will explain the matter-antimatter discoveries made at the
B-Factory, and their connection to this year's Nobel prize in physics.
It does not matter if you have no prior knowledge of Antimatter; just
bring your curiosity. Lecturer: Dr. Aaron Roodman, Stanford University.
mmmmmmmmmmmmmmmm
Quarks
Quarks are one type of matter
particle. Most of the matter we see around us is made from protons and
neutrons, which are composed of quarks.
There are six quarks, but physicists usually talk about them in terms of three pairs:
up/down, charm/strange,
and top/bottom. (Also, for each of these quarks, there is a corresponding
antiquark.) Be glad that quarks have such silly names -- it makes them easier
to remember!
Quarks have the unusual characteristic of having a fractional electric charge, unlike the
proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another
type of charge called color charge, which we will discuss later.
The most elusive quark, the top quark, was discovered in 1995
after its existence had been theorized for 20 years.
Want to see a particle physicist's idea of a good pun?
Charm
Up
Top
Down
Bottom
Strange
Cork model/bad pun by Don Groom, Particle Data Group, LBNL.
The naming of quarks...
...began when, in 1964, Murray Gell-Mann and George Zweig suggested
that hundreds of the particles known at the time could be explained as
combinations of just three fundamental particles. Gell-Mann chose the
name "quarks," pronounced "kworks," for these three particles, a nonsense word used by James Joyce in the novel Finnegan's Wake:
"Three quarks for Muster Mark!"
In order to make their calculations work, the quarks had to be assigned
fractional electrical charges of 2/3 and -1/3. Such charges had never
been observed before. Quarks are never observed by themselves, and so
initially these quarks were regarded as mathematical fiction.
Experiments have since convinced physicists that not only do quarks
exist, but there are six of them, not three.
How did quarks get their silly names?
There are six flavors of quarks. "Flavors" just means different kinds.
The two lightest are called up and downhe
The third quark is called strange. It was named after the "strangely" long lifetime of the K particle, the first composite particle found to contain this quark.
The fifth and sixth quarks were sometimes called truth and beauty in the past, but even physicists thought that was too cute.
The bottom quark was first discovered at Fermi National Lab (Fermilab) in 1977, in a composite particle called Upsilon ()
The top quark was discovered last,
also at Fermilab, in 1995. It is the most massive quark. It had been
predicted for a long time but had never been observed successfully until
then.
The Up and Down quarks are the most common and least massive quarks, being the constituents of protons and neutrons and thus of most ordinary matter. The fact that the free neutron decays
is thought to be the result of a more fundamental quark process
The Strange Quark: In 1947 during a study of cosmic ray interactions, a product of a
proton collision with a nucleus was found to live for much longer time
than expected: 10-10 seconds instead of the expected 10-23 seconds! This particle was named the lambda particle (Λ0) and the property which caused it to live so long was dubbed "strangeness" and that name stuck to be the name of one of the quarks from which the lambda particle is constructed. The lambda is a baryon which is made up of three quarks: an up, a down and a strange quark.
The omega-minus, a baryon composed of three strange quarks, is a classic example of the need for the property called "color" in describing particles. Since quarks are fermions with spin 1/2, they must obey the Pauli exclusion principle
and cannot exist in identical states. So with three strange quarks, the
property which distinguishes them must be capable of at least three
distinct values.
The Charm Quark: In 1974 a meson called the J/Psi particle was discovered. With a mass of 3100 MeV, over three times that of the proton, this particle was the first example of another quark, called the charm quark. The J/Psi is made up of a charm-anticharm quark pair.The lightest meson which contains a charm quark is the D meson.
It provides interesting examples of decay since the charm quark must be
transformed into a strange quark by the weak interaction in order for
it to decay.One baryon with a charm quark is a called a lambda with symbol
Λ+c . It has a composition udc and a mass of 2281 MeV/c2.
The Top Quark: Convincing evidence for the observation of the top quark was reported by Fermilab 's Tevatron
facility in April 1995. The evidence was found in the collision
products of 0.9 TeV protons with equally energetic antiprotons in the
proton-antiproton collider. The evidence involved analysis of trillions
of 1.8 TeV proton-antiproton collisions. The Collider Detector Facility group had found 56 top candidates over a predicted background of 23 and the D0 group
found 17 events over a predicted background of 3.8. The value for the
top quark mass from the combined data of the two groups after the
completion of the run was 174.3 +/- 5.1 GeV. This is over 180 times
the mass of a proton and about twice the mass of the next heaviest
fundamental particle, the Z0 vector boson at about 93 GeV.
The Bottom Quark: In 1977, an experimental group at Fermilab
led by Leon Lederman discovered a new resonance at 9.4 GeV/c^2 which
was interpreted as a bottom-antibottom quark pair and called the Upsilon meson.
Table of quark properties
Quarks and Leptons
are the building blocks which build up matter, i.e., they are seen as
the "elementary particles". In the present standard model, there are six
"flavors" of quarks. They can successfully account for all known mesons and baryons (over 200). The most familiar baryons are the proton and neutron,
which are each constructed from up and down quarks. Quarks are observed
to occur only in combinations of two quarks (mesons), three quarks
(baryons). There was a recent claim of observation of particles with
five quarks (pentaquark), but further experimentation has not borne it out.
*The masses should not be taken too seriously, because the confinement of quarks
implies that we cannot isolate them to measure their masses in a direct
way. The masses must be implied indirectly from scattering experiments.
The numbers in the table are very different from numbers previously
quoted and are based on the July 2010 summary in Journal of Physics G,
Review of Particle Physics, Particle Data Group. A summary can be found
on the LBL site.
These masses represent a strong departure from earlier approaches which
treated the masses for the U and D as about 1/3 the mass of a proton,
since in the quark model the proton has three quarks. The masses quoted
are model dependent, and the mass of the bottom quark is quoted for two
different models. But in other combinations they contribute different masses. In the pion, an up and an anti-down quark yield a particle of only 139.6 MeV of mass energy, while in the rho vector meson
the same combination of quarks has a mass of 770 MeV! The masses of C
and S are from Serway, and the T and B masses are from descriptions of
the experiments in which they were discovered.
Each of the six "flavors" of quarks can have three different "colors".
The quark forces are attractive only in "colorless" combinations of
three quarks (baryons), quark-antiquark pairs (mesons) and possibly
larger combinations such as the pentaquark that could also meet the
colorless condition. Quarks undergo transformations
by the exchange of W bosons, and those transformations determine the
rate and nature of the decay of hadrons by the weak interaction.
Armado
de sentido del humor y términos para el público en general, el ganador
del premio Nobel Murray Gell-Mann brinda a los seguidores de TED
conocimientos sobre física de partículas, mediante preguntas como: ¿Las
ecuaciones elegantes tienen más probabilidad de ser correctas que las no
elegantes?
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.[1] Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can only be found within hadrons or mesons.[2][3] For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
There are six types of quarks, known as flavors: up, down, strange, charm, bottom, and top.[4] Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay:
the transformation from a higher mass state to a lower mass state.
Because of this, up and down quarks are generally stable and the most
common in the universe, whereas strange, charm, top, and bottom quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators).
The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964.[5]
Quarks were introduced as parts of an ordering scheme for hadrons, and
there was little evidence for their physical existence until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968.[6][7] All six flavors of quark have since been observed in accelerator experiments; the top quark, first observed at Fermilab in 1995, was the last to be discovered.[5]
mmmmmmmmmmm
Hadrons, Baryons, and Mesons
Like social elephants, quarks only exist in groups with other quarks and are never found alone.
Composite particles made of quarks are called
HADRONS
Although individual quarks have fractional electrical charges, they combine such that
hadrons have a net integer electric charge. Another property of hadrons is that
they have no net color charge even though the quarks themselves carry color charge (we will talk more
about this later).
There are two classes of hadrons (try putting your mouse on the
elephants): Baryons and Mesons
BARYONS
...are any hadron which is made of three quarks (qqq).
Because
they are made of two up quarks and one down quark (uud), protons are baryons.
So are neutrons (udd).
MESONS
...contain one quark (q) and one antiquark ().
One example of a meson is a pion
(+),
which is made of an up quark and
a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (-)
is made up a down quark and an up antiquark.
Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K-) meson lives much
longer than most mesons, which is why it was
called "strange" and gave this name to the strange quark, one of its components.
A weird thing about hadrons is that only a very very very small part of the mass of a hadron is
due to the quarks in it. For example, a proton (uud) has more mass than the sum of the masses of its quarks:
Most of the mass we observe in a hadron comes from its kinetic and potential energy.
These energies are converted into the mass of the hadron as described by
Einstein's equation that relates energy and mass, E = mc2.
A baryon is a compositeparticle made up of three quarks (as distinct from mesons, which comprise one quark and one antiquark). Baryons and mesons belong to the hadronfamily, which are the quark-based particles. The name "baryon" comes from the Greek word for "heavy" (βαρύς, barys), because, at the time of their naming, most known particles had lower masses than the baryons.
As quark-based particles, baryons participate in the strong interaction, whereas leptons, which are not quark-based, do not. The most familiar baryons are the protons and neutrons that make up most of the mass of the visible matter in the universe. Electrons (the other major component of the atom) are leptons. Each baryon has a corresponding antiparticle
(antibaryon) where quarks are replaced by their corresponding
antiquarks. For example, a proton is made of two up quarks and one down
quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
Until recently, it was believed that some experiments showed the existence of pentaquarks — "exotic" baryons made of four quarks and one antiquark.[1][2] The particle physics community as a whole did not view their existence as likely in 2006,[3] and in 2008, considered evidence to be overwhelmingly against the existence of the reported pentaquarks.[4]
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about 2⁄3 the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few 100-millionths (10−8) of a second. Charged mesons decay (sometimes through intermediate particles) to form electrons and neutrinos. Uncharged mesons may decay to photons.
Mesons are not produced by radioactive decay, but appear in nature
only as short-lived products of very high-energy interactions in matter,
between particles made of quarks. In cosmic ray
interactions, for example, such particles are ordinary protons and
neutrons. Mesons are also frequently produced artificially in
high-energy particle accelerators that collide protons, anti-protons, or
other particles containing quarks.
In nature, the importance of lighter mesons is that they are the associated quantum-field particles that transmit the nuclear force,
in the same way that photons are the particles that transmit the
electromagnetic force. The higher energy (more massive) mesons were
created momentarily in the Big Bang
but are not thought to play a role in nature today. However, such
particles are regularly created in experiments, in order to understand
the nature of the heavier types of quark which compose the heavier
mesons.
mmmmmmmmmmmmm
Leptons
The other type of matter particles are the leptons.
There are six leptons, three of which have electrical charge and three of which do not.
They appear to be point-like particles without internal structure.
The best known lepton is the electron (e-). The other two
charged leptons are the muon() and the
tau(), which are charged like electrons but
have a lot more mass.
The other leptons are the three types of neutrinos
(). They have no electrical charge, very little mass, and they
are very hard to find.
Quarks are sociable and only exist in composite particles with other quarks, whereas leptons
are solitary particles. Think of the charged leptons as
independent cats with associated neutrino fleas, which are very hard to see.
For each lepton there is a corresponding antimatter antilepton. Note that the anti-electron has
a special name, the "positron."
Trivia: "Lepton" comes from the Greek for "small mass," but this is a misnomer. Why?
[Answer]
Answer: Even though "lepton" comes from the Greek for "small mass", the tau lepton is more than
3000 times as massive as the electron.
Lepton decays
The heavier leptons, the muon and the tau,
are not found in ordinary matter at all. This is
because when they are produced they very quickly decay, or transform, into lighter leptons.
Sometimes the tau lepton will decay into a quark, an antiquark, and a tau neutrino. Electrons and the three kinds of
neutrinos are stable and thus the types we commonly see around us.
When a heavy lepton decays, one of the particles it decays into is always its corresponding
neutrino. The other particles could be a quark and its antiquark, or another lepton and its
antineutrino.
Physicists have observed that some types of
lepton decays are possible and some are not. In order to explain this, they divided the
leptons into three lepton families: the electron and its neutrino, the muon and its neutrino, and
the tau and its neutrino. The
number of members in each family must remain constant in a decay.
(A particle and an antiparticle in the same family "cancel out"
to make the total of them equal zero.)
Although leptons are solitary, they are always loyal to their families!
Lepton type conservation
Leptons are divided into three lepton families: the electron and its neutrino, the muon and its neutrino, and
the tau and its neutrino.
We use the terms "electron number," "muon number," and "tau number" to refer to the lepton family of
a particle. Electrons and their neutrinos have electron number +1, positrons and their antineutrinos
have electron number -1, and all other particles have electron number 0. Muon number and tau number
operate analogously with the other two lepton families.
One important thing about leptons, then, is that electron number, muon number, and tau number are
always conserved when a massive lepton decays into smaller ones.
Let's take an example decay.
A muon decays into a muon neutrino, an electron, and an electron antineutrino:
As you can see, electron, muon, and tau numbers are conserved. These and other conservation laws
are what we believe define whether or not
a given hypothetical lepton decay is possible.
Lepton decay quiz
Which lepton decays are possible? Why or why not?
(A tau lepton decays into an electron, an electron antineutrino, and a tau neutrino.)
Yes! Charge, tau number, electron number, and energy are all conserved.
(A tau lepton decays into a muon and a tau neutrino.)
No! Muon number is not conserved. A muon has a muon number of 1, and
thus the right side of the decay equation has muon number 1, but the
left side has muon number of 0.
Now try a tricky one:
(An electron decays into a muon, a muon antineutrino, and an electron neutrino.) No! Surprise! Although electron and muon numbers are both conserved,
energy is not conserved. A muon has a lot more mass than an electron,
and a lepton cannot decay into something more massive than it started
out!
A lepton is an elementary particle and a fundamental constituent of matter.[1] The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed.
There are six types of leptons, known as flavours, forming three generations.[2] The first generation is the electronic leptons, comprising the electron (e−) and electron neutrino (ν
e); the second is the muonic leptons, comprising the muon (μ−) and muon neutrino (ν
μ); and the third is the tauonic leptons, comprising the tau (τ−) and the tau neutrino (ν
τ). Electrons have the least mass of all the charged
leptons. The heavier muons and taus will rapidly change into electrons
through a process of particle decay:
the transformation from a higher mass state to a lower mass state. Thus
electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).
Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction. For every lepton flavor there is a corresponding type of antiparticle, known as antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to certain theories, neutrinos may be their own antiparticle, but it is not currently known whether this is the case or not.
The first charged lepton, the electron, was theorized in the mid-19th century by several scientists[3][4][5] and was discovered in 1897 by J. J. Thomson.[6] The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, but it was erroneously classified as a meson at the time.[7]
After investigation, it was realized that the muon did not have the
expected properties of a meson, but rather behaved like an electron,
only with higher mass. It took until 1947 for the concept of "leptons"
as a family of particle to be proposed.[8] The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay.[8] It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956.[8][9] The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz and Jack Steinberger,[10] and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory.[11] The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery.[12][13]
Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons. Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium.
mmmmmmmmmmmmmmmmmmmmmmmmmmmmmm
Neutrinos
Neutrinos are, as we've said, a type of lepton.
Since they have no electrical or strong charge they almost never interact
with any other particles. Most neutrinos pass right through the earth without ever
interacting with a single atom of it.
Neutrinos are produced in a variety of interactions,
especially in particle decays. In fact, it was through a careful study of radioactive decays that
physicists hypothesized the neutrino's existence.
For example:(1) In a radioactive nucleus, a neutron at rest (zero momentum) decays,
releasing a proton and an electron. (2) Because of the
law of conservation of momentum, the resulting products of the
decay must have a total momentum of zero, which the observed proton and electron
clearly do not.
(3) Therefore, we need to infer
the presence of another particle with appropriate momentum to balance the event.
(4) We hypothesize that an antineutrino
was released; experiments have confirmed that this is indeed what happens. Because neutrinos were produced in great abundance
in the early universe and rarely interact with matter, there are a lot of them in the Universe. Their tiny mass but huge numbers
may contribute to total mass of the universe and affect its expansion.
Quiz - What particles are made of
Questions:
What are protons made of? Protons are made of two up quarks and one down quark, expressed as uud.
What are electrons made of? Nothing! Electrons are fundamental, as far as we know.
Neutrinos (Wikipedia)
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle[1] with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected. The neutrino (meaning "small neutral one" in Italian) is denoted by the Greek letter ν (nu).
Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are affected only by the weak sub-atomic force, of much shorter range than electromagnetism, and gravity,
which is relatively weak on the subatomic scale, and are therefore able
to travel great distances through matter without being affected by it.
Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Each type also has a corresponding antiparticle, called an antineutrino with an opposite chirality.
Most neutrinos passing through the Earth emanate from the Sun. About 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.[2]
In September 2011, neutrinos apparently moving faster than light were detected (see OPERA neutrino anomaly).
Since then the experiment has undergone extensive critique and efforts
to replicate the results because confirming the results would change our
understanding of the theory of relativity. In November 2011, the experiment was refined and yielded the same result. (See Speed below)
Covering the 150 million km journey that
a neutrino takes from the centre of the Sun to the Earth, and how we
detect them. Don't forget to visit http://www.sciencemadefun.org.uk
Follow Science Made Fun founder Colin on Twitter for more science goodness! http://www.twitter.com/skyponderer
Which of the following are made of quarks?
Baryons? Yes, they are made of three quarks put together.
Mesons? Yes, they are made of one quark and one antiquark.
Barons?Yes, even members of the English nobility are made of quarks.
mmmmmmmmmmmm
The Standard Model - What holds it together? - The four interactions -
Now we think we have a good idea of what the world is made of: quarks and leptons. So...
What holds it together?
The universe, which we know and love,
exists because the fundamental particles interact. These interactions
include attractive and repulsive forces, decay, and annihilation.
There are four fundamental interactions between particles, and all forces
in the world can be attributed to these four interactions!
That's right: Any force you can think of -- friction, magnetism, gravity, nuclear decay, and
so on -- is caused by one of these four fundamental interactions.
What's the difference between a force and an interaction?
This is a hard distinction to make. Strictly speaking, a force is the effect on a particle
due to the presence of other particles. The interactions of a particle include all the
forces that affect it, but also include decays and annihilations that the particle might go through.
(We will spend the next chapter discussing these decays and annihilations in more depth.)
The reason this gets confusing is that most people, even most physicists, usually use "force" and
"interaction" interchangeably, although "interaction" is more correct. For instance, we call the
particles which carry the interactions force carrier particles. You will usually be okay
using the terms interchangeably, but you should know that they are different.
One tricky question that plagued physicists for many years was...
How do matter particles interact?
The problem is that things interact without touching! How do two magnets "feel" each other's
presence and attract or repel accordingly? How does the sun attract the earth?
We know the answers to these questions are "magnetism" and "gravity," but what are these forces?
At a fundamental level, a force isn't just something that happens to particles. It is
a thing which is passed between two particles.
The unseen effect
You can think about forces as being analogous to the following situation: (Flash)
Two people are standing on an ice pond. One person moves their arm and is pushed backwards;
a moment later the other person grabs at an invisible object
and is driven backwards. Even though you cannot see a basketball, you can assume that one person threw
a basketball to the other person because you see its effect on the people. (Click on the checkmark or cross below the
animation in order to make the basketball appear or disappear.)
It turns out that all interactions which affect matter particles are due to an exchange of
force carrier particles, a different type of particle altogether. These particles are like
basketballs tossed between matter particles (which are like the basketball players).
What we normally think of as "forces" are actually the effects of force carrier particles
on matter particles.
The basketball animation is, of course, a very crude analogy since it can only explain
repulsive forces and gives no hint of how exchanging particles
can result in attractive forces.
We see examples of attractive forces in everyday life (such as magnets and
gravity), and so we generally take it for granted that an
object's presence can just affect another object. It is when we approach
the deeper question, "How can two objects affect one another without
touching?" that we propose that the invisible force could be an
exchange of force carrier particles. Particle physicists have found that we can
explain the force of one particle acting on another to INCREDIBLE
precision by the exchange of these force carrier particles.
One important thing to know about force carriers is that a particular force carrier particle can
only be absorbed or produced by a matter particle which is affected by that particular force. For instance,
electrons and protons have electric charge, so they can produce and absorb the electromagnetic
force carrier, the photon. Neutrinos, on the other hand, have no electric charge, so they cannot
absorb or produce photons.
ELECTROMAGNETISM (E-M force)
The electromagnetic force causes like-charged things to repel and oppositely-charged things to
attract. Many everyday forces, such as friction, and even magnetism, are caused by the
electromagnetic, or E-M force. For instance, the force that keeps you from falling through the floor
is the electromagnetic force which causes the atoms making up the matter in your feet and the
floor to resist being displaced.
The carrier particle of the electromagnetic force is the photon ().
Photons
of different energies span the electromagnetic
spectrum of x rays, visible light, radio waves, and so forth.
Photons have zero mass, as far as we know, and always travel at the "speed of light", c, which is
about 300,000,000 meters per second, or 186,000 miles per second, in a vacuum.
Residual EM force
Atoms usually have the same numbers of protons and electrons. They are electrically neutral,
therefore, because the positive protons cancel out the negative electrons. Since they are neutral,
what causes them to stick together to form stable molecules?
The answer is a bit strange: we've discovered that the charged parts of one atom can interact
with the charged parts of another atom. This allows different atoms to bind together, an effect
called the residual electromagnetic force.
So the electromagnetic force is what allows atoms to bond and form molecules, allowing the world
to stay together and create the matter you interact with all of the time. Amazing, isn't it?
All the
structures of the world exist simply because protons and electrons have opposite charges!
See? Now you know the
meaning of life!
The Meaning of Life: Life is just a neat example of electromagnetic force!
mmmm
What about the nucleus?
We have another problem with atoms, though. What binds the nucleus together?
The nucleus of an atom consists of a bunch of protons and neutrons crammed together.
Since neutrons have no charge and the positively-charged protons repel one another,
why doesn't the nucleus blow apart?
We cannot account for the nucleus staying together with just electromagnetic force.
What else could there be? Gravity? Nope! The gravitational force is far too weak to overpower the
electromagnetic force.
So how can we account for this dilemma?
Strong
STRONG
To understand what is happening inside the nucleus, we need to understand
more about the quarks that make up the protons and neutrons in the nucleus.
Quarks have electromagnetic charge, and they also have an altogether
different kind of
charge called color charge. The force between color-charged particles
is very strong, so this force is "creatively" called
STRONG
The strong force holds quarks together to form hadrons, so its carrier particles are
whimsically called
gluons because they so tightly "glue" quarks together.
(Other name candidates included the "hold-on," the "duct-tape-it-on," and the "tie-it-on!")
Color charge behaves differently than electromagnetic charge.
Gluons, themselves,
have color charge, which is weird and not at all like photons which
do not have
electromagnetic charge. And while quarks have color charge, composite particles made out of
quarks have no net color charge (they are color neutral).
For this reason, the strong force only takes place on the really small level of quark interactions,
which is why you are not aware of the strong force in your everyday life.
Strong interaction (Wikipedia)
In particle physics, the strong interaction (also called the strong force, strong nuclear force, or color force) is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. At atomic scale, it is about 100 times stronger than electromagnetism, which in turn is orders of magnitude stronger than the weak force interaction and gravitation.
The strong interaction is observable in two areas: on a larger scale (about 1 to 3 femtometers (fm)), it is the force that binds protons and neutrons together to form the nucleus of an atom. On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is also the force (carried by gluons) that holds quarks together to form protons, neutrons and other hadron particles.
In the context of binding protons and neutrons (nucleons) together to form atoms, the strong interaction is called the nuclear force (or residual strong force).
In this case, it is the residuum of the strong interaction between the
quarks that make up the protons and neutrons. As such, the residual
strong interaction obeys a quite different distance-dependent behavior
between nucleons, from when it is acting to bind quarks within nucleons.
The strong interaction is thought to be mediated by gluons, acting upon quarks, antiquarks,
and other gluons. Gluons, in turn, are thought to interact with quarks
and gluons because all carry a type of charge called "color charge."
Color charge is analogous to electromagnetic charge, but it comes in
three types not two, and it results in a different type of force, with
different rules of behavior. These rules are detailed in the theory of quantum chromodynamics (QCD), which is the theory of quark-gluon interactions.
Confinement,
which means that the force between quarks does not diminish as they are
separated. Because of this, it would take an infinite amount of energy
to separate two quarks; they are forever bound into hadrons such as the proton and the neutron. Although analytically unproven, confinement is widely believed to be true because it explains the consistent failure of free quark searches, and it is easy to demonstrate in lattice QCD.
There is no known phase-transition line separating these two
properties; confinement is dominant in low-energy scales but, as energy
increases, asymptotic freedom becomes dominant. Unsolved problems in physics QCD in the non-perturbative regime:
Confinement: the equations of QCD remain unsolved at energy scales relevant for describing atomic nuclei. How does QCD give rise to the physics of nuclei and nuclear constituents?
Quarks and gluons are color-charged particles.
Just as electrically-charged particles interact by
exchanging photons in electromagnetic interactions, color-charged particles exchange gluons in
strong interactions.
When two quarks are close to one another, they exchange gluons and
create a very strong
color force field that binds the quarks together. The force
field gets stronger as the quarks get further apart.
Quarks constantly change their color charges as they
exchange gluons with other quarks.
How does color charge work?
There are three color charges and three corresponding anticolor
(complementary color) charges. Each quark has one of the three color charges and each antiquark has one of
the three anticolor charges. Just as a mix of red, green, and blue light yields
white light, in a baryon a combination of "red," "green," and "blue" color charges is color neutral, and in an
antibaryon "antired," "antigreen," and "antiblue" is also color neutral. Mesons are color neutral because they carry
combinations such as "red" and "antired."
Because gluon-emission and -absorption always changes color, and -in
addition - color is a conserved quantity - gluons
can be thought of as carrying a color and an anticolor charge. Since
there are nine possible color-anticolor combinations we might
expect nine different gluon charges, but the mathematics works out such
that there are only eight combinations.
Unfortunately, there is no intuitive explanation for this result.
Important Disclaimer:
"Color charge" has nothing to do with the
visible colors, it is just a convenient naming convention for a mathematical
system physicists developed to explain their observations about quarks in hadrons.
Color charge (Wikipedia)
In particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). Color charge has analogies with the notion of electric charge
of particles, but because of the mathematical complications of QCD,
there are many technical differences. The "color" of quarks and gluons
is completely unrelated to visual perception of color.[1] Rather, it is a name for a property that has almost no manifestation at distances above the size of an atomic nucleus. The term color was chosen because the abstract property to which it refers has three aspects, which are analogized to the three primary colors of red, green, and blue.[2] By comparison, the electromagnetic charge has a single aspect, which takes the values positive or negative.
Shortly after the existence of quarks was first proposed in 1964, Oscar W. Greenberg introduced the notion of color charge to explain how quarks could coexist inside some hadrons in otherwise identical quantum states without violating the Pauli exclusion principle. The theory of quantum chromodynamics has been under development since the 1970s and constitutes an important component of the Standard Model of particle physics.
Red, green, and blue
In QCD, a quark's color can take one of three values, called red,
green, and blue. An antiquark can take one of three anticolors, called
antired, antigreen, and antiblue (represented as cyan, magenta and
yellow, respectively). Gluons are mixtures of two colors, such as red
and antigreen, which constitutes their color charge. QCD considers eight
gluons of the possible nine color-anticolor combinations to be unique;
see eight gluon colors for an explanation.
The following illustrates the coupling constants for color-charged particles:
The quark colors (red, green, blue) combine to be colorless
The quark anticolors (antired, antigreen, antiblue) also combine to be colorless
A hadron with 3 quarks (red, green, blue) before a color change
Red quark emits a red-antigreen gluon
Green quark has absorbed the red-antigreen gluon and is now red; color is conserved
An animation of the interaction inside a neutron
- Quark confinement -
Color-charged particles cannot be found individually. For this reason, the
color-charged quarks are confined in groups (hadrons) with other
quarks. These composites are color neutral.
The development of the Standard Model's theory of the strong
interactions reflected evidence that quarks combine only into
baryons (three quark objects), and mesons (quark-antiquark objects),
but not, for example, four-quark objects. Now we understand that only
baryons (three different colors) and mesons (color and anticolor) are color-neutral.
Particles such as ud or uddd that
cannot be combined into color-neutral states are never observed.
Color confinement (Wikipedia) Color confinement, often simply called confinement, is the physics phenomenon that color charged particles (such as quarks) cannot be isolated singularly, and therefore cannot be directly observed.[1] Quarks, by default, clump together to form groups, or hadrons. The two types of hadrons are the mesons (one quark, one antiquark) and the baryons
(three quarks). The constituent quarks in a group cannot be separated
from their parent hadron, and this is why quarks can never be studied or
observed in any more direct way than at a hadron level.[2]
The color force favors confinement because at a certain range it is more
energetically favorable to create a quark-antiquark pair than to
continue to elongate the color flux tube. This is analoguous to the
behavior of an elongated rubber-band.
An animation of color confinement. Energy is supplied to the quarks, and
the gluon tube elongates, till it reaches a point where it "snaps" and
forms a quark-antiquark pair.
Color-Force Field
The quarks in a given hadron madly exchange gluons. For this reason,
physicists talk about the color-force field which consists of the
gluons holding the bunch of quarks together.
If one of the quarks in a
given hadron is pulled away from its neighbors, the
color-force field "stretches" between that
quark and its neighbors. In so doing,
more and more energy is added to the color-force field as the quarks are
pulled apart. At some point, it is energetically
cheaper for the color-force field
to "snap" into a new quark-antiquark pair. In so doing, energy is conserved because the
energy of the color-force field is
converted into the mass of the new quarks,
and the color-force field can "relax" back to an unstretched state.
Quarks cannot exist individually because the color force increases as they are pulled apart.
- Quarks emit gluons -
Color charge is always conserved.
When a quark emits or absorbs a gluon, that
quark's color must change in order to conserve color charge.
For example, suppose a red quark
changes into a blue quark and emits a red/antiblue gluon (the image below illustrates antiblue as yellow).
The net color is still red. This is because - after the emission of the gluon - the blue
color of the quark cancels with the antiblue color of the gluon. The remaining color then is the
red color of the gluon.
Quarks emit and absorb gluons very frequently within a hadron,
so there is no way to observe the color of an individual quark.
Within a hadron, though, the color of the two quarks exchanging a gluon
will change in a way that keeps the bound system in a color-neutral
state.
A quark and an antiquark (red color) are glued together (green color) to
form a meson (result of a lattice QCD simulation by M. Cardoso et al.
- Residual strong force -
So now we know that the strong force binds quarks together because quarks have
color charge. But that still does not explain what holds the nucleus together, since positive
protons repel each other with electromagnetic force,
and protons and neutrons are
color-neutral.
So what holds the nucleus together? Huh?
The answer is that, in short, they don't call it the strong force for nothing.
The strong force between the quarks in one proton and the quarks in another proton
is strong enough to overwhelm the repulsive electromagnetic force.
This is called the residual strong interaction, and it is what "glues" the nucleus
together.
mmm
The Standard Model - What holds it together? - Weak -
WEAK
There are six kinds of quarks and six kinds of leptons. But all the stable matter of the universe
appears to be made of just the two least-massive quarks
(up quark and down quark), the least-massive
charged lepton (the electron), and the neutrinos.
Weak interactions are responsible for the decay of massive quarks and leptons
into lighter quarks
and leptons. When fundamental particles decay, it is very strange: we observe the particle vanishing and being replaced
by two or more different particles. Although the total of mass and energy is conserved, some of the original
particle's mass is converted into kinetic energy, and the resulting particles always have less mass than
the original particle that decayed.
The only matter around us that is stable is made up
of the smallest quarks and leptons, which cannot decay any further.
When a quark or lepton changes type (a muon changing to an electron, for instance)
it is said to change flavor.
All flavor changes are due to the weak interaction.
Each of the quarks has a different "flavor,"
which is just the term physicists use to distinguish between the six
types of quarks. For instance, the flavor of an up quark is simply "up."
Charged weak interactions can change the flavor of a particle! And only
charged weak interactions can do this. Weak interactions which involve
the neutral Z particle cannot change a particle's flavor.
Leptons also have a "flavor." In addition, they have electron number,
muon number, and tau number, as discussed earlier. While lepton flavor
is changed by weak interactions, the process conserved electron, muon,
and tau numbers.
The carrier particles of the weak interactions are the W+, W-, and the Z particles.
The W's are electrically charged and the Z is neutral.
The Standard Model has united electromagnetic interactions and weak interactions into one
unified interaction called electroweak.
Weak interaction (Wikipedia) Weak interaction (often called the weak force or sometimes the weak nuclear force), is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars. Weak interactions affect all known fermions; that is, particles whose spin (a property of all particles) is a half-integer.
In the Standard Model of particle physics the weak interaction is theorised as being caused by the exchange (i.e., emission or absorption) of W and Z bosons; and because it is a consequence of the emission (or absorption) of bosons it is a non-contact force. The best known effect of this emission is beta decay, a form of radioactivity.
The Z and W bosons are much heavier than protons or neutrons and it is
the heaviness that accounts for the very short range of the weak
interaction. It is termed weak because its typical field strength is several orders of magnitude less than that of both electromagnetism and the strong nuclear force. Most particles will decay by a weak interaction over time. It has one unique property – namely quark flavour changing – that does not occur in any other interaction. In addition, it also breaks parity-symmetry and CP-symmetry. Quark flavour changing allows for quarks to swap their 'flavour', one of six, for another.
The weak force was originally described, in the 1930s, by Fermi's theory of a contact four-fermion interaction: which is to say, a force with no range (i.e., entirely dependent on physical contact[1]).
However, it is now best described as a field, having range, albeit a
very short range. In 1968, the electromagnetic force and the weak
interaction were unified, when they were shown to be two aspects of a
single force, now termed the electro-weak force.
Weak interactions are most noticeable when particles undergo beta decay
and in the production of deuterium and then helium from hydrogen that
powers the sun's thermonuclear process. Such decay also makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14. It can also create radioluminescence, commonly used in tritium illumination, and in the related field of betavoltaics.[2]
A diagram plotting mass against charge for the six quarks of the
standard model, and depicting the various decay routes due to the weak
interaction and some indication of their likelihood.
The W and Z bosons (together known as the weak bosons) are the elementary particles that mediate the weak interaction; their symbols are W+, W− and Z. The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticle. The Z boson is electrically neutral and its own antiparticle. All three of these particles are very short-lived with a half-life of about 3×10−25 s. Their discovery was a major success for what is now called the Standard Model of particle physics.
The W bosons are named after the weak force. The physicistSteven Weinberg named the additional particle the "Z particle"[3],
later giving the explanation that it was the last additional particle
needed by the model – the W bosons had already been named – and that it
has zero electric charge.[4]
The two W bosons are best known as mediators of neutrino absorption and emission, where their charge is associated with electron or positron emission or absorption, always causing nuclear transmutation. The Z boson is most easily detected as a necessary theoretical force-mediator, whenever neutrinos scatter elastically
from matter, something that must happen without production or
absorption of new charged particles. Such behavior (which is almost as
common as inelastic neutrino interactions) is seen in bubble chambers
irradiated with neutrino beams. Whenever an electron simply "appears" in
such a chamber as a free particle, and begins to move as a result of an
impulse in the direction of the neutrinos, and this behavior happens
more often when the neutrino beam is present, it is inferred to be a
result of a neutrino interacting directly with the electron. Such an
interaction can only happen via the weak force. Since such an electron
is not created from a nucleon, and remains unchanged except for the
impulse imparted by the neutrino, this weak force interaction between
the neutrino and the electron must be mediated by a particle with no
charge, and so must be mediated by a Z boson.
The Standard Model - What holds it together? - Electroweak -
Unified ElectroWeak Theory
In the Standard Model the weak and the electromagnetic interactions have been combined into a
unified electroweak theory.
Physicists had long believed that weak forces were closely related to electromagnetic forces.
Eventually they discovered that at very short distances (about 10-18 meters)
the strength of the weak interaction is comparable to that of the electromagnetic. On the other
hand, at thirty times that distance (3x10-17 m) the strength of the weak interaction is 1/10,000th
than that of the electromagnetic interaction. At distances typical for quarks in a proton or neutron (10-15 m) the
force is even tinier.
Physicists concluded that, in fact, the weak and electromagnetic forces
have essentially
equal strengths.
This is because the strength of the interaction depends
strongly on both the mass of the force carrier and
the distance of the interaction.
The difference between their observed strengths is due to the huge difference in
mass between the W and Z particles, which are very massive, and the photon, which has no mass as
far as we know. Electro-Weak force (Wikipedia)
In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction.
Although these two forces appear very different at everyday low
energies, the theory models them as two different aspects of the same
force. Above the unification energy, on the order of 100 GeV, they would merge into a single electroweak force. Thus if the universe is hot enough (approximately 1015K, a temperature exceeded until shortly after the Big Bang) then the electromagnetic force and weak force will merge into a combined electroweak force.
For contributions to the unification of the weak and electromagnetic interaction between elementary particles, Abdus Salam, Sheldon Glashow and Steven Weinberg were awarded the Nobel Prize in Physics in 1979. The existence of the electroweak interactions was experimentally established in two stages, the first being the discovery of neutral currents in neutrino scattering by the Gargamelle collaboration in 1973, and the second in 1983 by the UA1 and the UA2 collaborations that involved the discovery of the W and Zgauge bosons in proton-antiproton collisions at the converted Super Proton Synchrotron.
Electroweak Unification
The discovery of the W and Z particles, the intermediate vector bosons,
in 1983 brought experimental verification of particles whose prediction
had already contributed to the Nobel prize awarded to Weinberg, Salam, and
Glashow in 1979. The photon , the particle involved in the electromagnetic
interaction, along with the W and Z provide the necessary pieces to unify
the weak and electromagnetic
interactions. With masses around 80 and 90 Gev, respectively,
the W and Z were the most massive particles seen at the time of
discovery while the photon is massless. The difference in masses is
attributed to spontaneous symmetry breaking as the hot universe cooled. The theory suggests that at very high temperatures where the equilibrium kT energies
are in excess of 100 GeV, these particles are essentially identical and
the weak and electromagnetic interactions were manifestations of a
single force. The question of how the W and Z got so much mass in the
spontaneous symmetry breaking is still a perplexing one. The
symmetry-breaking mechanism is called a Higgs field, and requires a new
boson, the Higgs boson to mediate it.
The next step is the inclusion of the strong interaction in what is called grand unification.
mmmmmmmmmmmmmmmmmm
The Standard Model - What holds it together? - Gravity -
What about gravity?
Gravity is weird. It is clearly one of the fundamental interactions, but the Standard
Model cannot satisfactorily explain it. This is one of those
major unanswered problems in physics today.
In addition, the gravity force carrier particle has not been found. Such
a particle, however, is predicted to exist and may someday be found: the graviton.
Fortunately, the effects of gravity are extremely tiny in most particle physics
situations compared to the
other three interactions, so theory and experiment can be compared without including
gravity in the calculations. Thus, the Standard Model works without explaining
gravity.
We know how to calculate gravitational forces, but we do not know
how to integrate gravity into the mathematics of the quantum theory of the Standard Model. (The fact
that we have not seen the graviton yet is not
a surprise in the Standard Model, because the graviton has extremely
weak interactions, so is rarely produced and rarely detected.)
In the same way that Isaac Newton's laws of mechanics were not wrong but
needed to be extended by Einstein to be more accurate about very high velocities, we need
to extend the Standard Model with a new theory that will explain
gravity thoroughly.
Gravity (Wikipedia) Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a forceproportional to their mass. Gravitation is most familiar as the agent that gives weight
to objects with mass and causes them to fall to the ground when
dropped. Gravitation causes dispersed matter to coalesce, and coalesced
matter to remain intact, thus accounting for the existence of the Earth, the Sun, and most of the macroscopic objects in the universe.
Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth; for the formation of tides; for natural convection, by which fluid flow occurs under the influence of a density gradient
and gravity; for heating the interiors of forming stars and planets to
very high temperatures; and for various other phenomena observed on
Earth.
Gravitation is one of the four fundamental interactions of nature, along with electromagnetism, and the nuclear strong force and weak force. Modern physics describes gravitation using the general theory of relativity by Einstein, in which it is a consequence of the curvature of spacetime governing the motion of inertial objects. The simpler Newton's law of universal gravitation provides an accurate approximation for most physical situations.
Gravitation keeps the planets in orbits around the Sun. (Not to scale)
Newton's law of universal gravitation: In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law
of universal gravitation. In his own words, “I deduced that the forces
which keep the planets in their orbs must [be] reciprocally as the
squares of their distances from the centers about which they revolve:
and thereby compared the force requisite to keep the Moon in her Orb
with the force of gravity at the surface of the Earth; and found them
answer pretty nearly.” The Equivalence Principle: explored by a succession of researchers including Galileo, Loránd Eötvös,
and Einstein, expresses the idea that all objects fall in the same way.
The simplest way to test the weak equivalence principle is to drop two
objects of different masses
or compositions in a vacuum, and see if they hit the ground at the same
time. These experiments demonstrate that all objects fall at the same
rate when friction (including air resistance) is negligible. The equivalence principle can be used to make physical deductions about the gravitational constant, the geometrical nature of gravity, the possibility of a fifth force, and the validity of concepts such as general relativity and Brans-Dicke theory. Introduction to general relativity:General relativity (GR) is a theory of gravitation that was developed by Albert Einstein between 1907 and 1915. According to general relativity, the observed gravitational attraction between masses results from their warping of space and time.
High-precision test of general relativity by the Cassini space probe (artist's impression): radio signals sent between the Earth and the probe (green wave) are delayed by the warping of space and time (blue lines) due to the Sun's mass.
In general relativity, the effects of gravitation are ascribed to spacetimecurvature instead of a force. The starting point for general relativity is the equivalence principle,
which equates free fall with inertial motion, and describes
free-falling inertial objects as being accelerated relative to
non-inertial observers on the ground.[7][8] In Newtonian physics, however, no such acceleration can occur unless at least one of the objects is being operated on by a force.
Einstein proposed that spacetime is curved by matter, and that
free-falling objects are moving along locally straight paths in curved
spacetime. These straight paths are called geodesics.
Like Newton's first law of motion, Einstein's theory states that if a
force is applied on an object, it would deviate from a geodesic. For
instance, we are no longer following geodesics while standing because
the mechanical resistance of the Earth exerts an upward force on us, and
we are non-inertial on the ground as a result. This explains why moving
along the geodesics in spacetime is considered inertial.
Einstein discovered the field equations of general relativity, which relate the presence of matter and the curvature of spacetime and are named after him. The Einstein field equations are a set of 10 simultaneous, non-linear, differential equations. The solutions of the field equations are the components of the metric tensor
of spacetime. A metric tensor describes a geometry of spacetime. The
geodesic paths for a spacetime are calculated from the metric tensor.
mmmmmmm
The Standard Model - What holds it together?
- Interaction summary -
The Standard Model - What holds it together? - Interaction summary
This is a summary of the different interactions, their force carrier particles, and what particles
they act on:
Which fundamental interaction is responsible for:
Friction? Friction is caused by residual electromagnetic interactions between the atoms of the two materials.
Nuclear bonding? Nuclear bonding is caused by residual strong interactions between the various parts of the nucleus.
Planetary orbits? The planets orbit because of the gravity that attracts them to the sun!
Even though gravity is a relatively weak force, it still has very
important effects on the world.
Other questions:
Which interactions act on neutrinos? Weak and Gravity
Which interaction has heavy carriers? Weak (W+, W-, and Z)
Which interactions act on the protons in you? All of them.
Which force carriers cannot be isolated? Why? Gluons, because they carry color charge themselves.
Which force carriers have not been observed? Gravitons (Gluons have been observed indirectly.)
The electromagnetic force holds atoms and molecules together. In
fact, the forces of electric attraction and repulsion of electric
charges are so dominant over the other three fundamental forces that
they can be considered to be negligible as determiners of atomic and
molecular structure. Even magnetic effects are usually apparent only at
high resolutions, and as small corrections.
The Strong Force
A force which can hold a nucleus together against the enormous
forces of repulsion of the protons is strong indeed. However, it is not
an inverse square force like the electromagnetic force and it has a very
short range. Yukawa modeled the strong force as an exchange force in which the exchange particles are pions and other heavier particles. The range of a particle exchange force is limited by the uncertainty principle. It is the strongest of the four fundamental forces
Since the protons and neutrons which make up the nucleus are themselves considered to be made up of quarks, and the quarks are considered to be held together by the color force,
the strong force between nucleons may be considered to be a residual
color force. In the standard model, therefore, the basic exchange
particle is the gluon
which mediates the forces between quarks. Since the individual gluons
and quarks are contained within the proton or neutron, the masses
attributed to them cannot be used in the range relationship to predict
the range of the force. When something is viewed as emerging from a
proton or neutron, then it must be at least a quark-antiquark pair, so
it is then plausible that the pion as the lightest meson should serve as
a predictor of the maximum range of the strong force between nucleons.
The sketch is an attempt to show one of many forms the gluon
interaction between nucleons could take, this one involving up-antiup
pair production and annililation and producing a π- bridging the nucleons.
The Weak Force
One of the four fundamental forces, the weak interaction involves the exchange of the intermediate vector bosons, the W and the Z. Since the mass of these particles is on
the order of 80 GeV, the uncertainty principle dictates a range of about 10-18 meters which is about 0.1% of the diameter of a proton.
The weak interaction changes one flavor of quark into another. It is crucial to the structure of the universe in that
1. The sun would not burn without it since the weak interaction causes the
transmutation p -> n so that deuterium can form and deuterium fusion can take place.
2. It is necessary for the buildup of heavy nuclei.
The role of the weak force in the transmutation of quarks
makes it the interaction involved in many decays of nuclear particles
which require a change of a quark from one flavor to another. It was in
radioactive decay such as beta decay
that the existence of the weak interaction was first revealed. The
weak interaction is the only process in which a quark can change to
another quark, or a lepton to another lepton - the so-called "flavor
changes".
The discovery of the W and Z particles in 1983 was hailed as a confirmation of the theories which connect the weak force to the electromagnetic force in electroweak unification.
The weak interaction acts between both quarks and leptons, whereas
the strong force does not act between leptons. "Leptons have no color,
so they do not participate in the strong interactions; neutrinos have no
charge, so they experience no electromagnetic forces; but all of them join in the weak interactions."(Griffiths)
Gravity is the weakest of the four fundamental forces,
yet it is the dominant force in the universe for shaping the large
scale structure of galaxies, stars, etc. The gravitational force between
two masses m1 and m2 is given by the relationship:
This is often called the "universal law of gravitation" and G the universal gravitation constant. It is an example of an inverse square law
force. The force is always attractive and acts along the line joining
the centers of mass of the two masses. The forces on the two masses are
equal in size but opposite in direction, obeying Newton's third law. Viewed as an exchange force, the massless exchange particle is called the graviton.
The gravity force has the same form as Coulomb's law
for the forces between electric charges, i.e., it is an inverse square
law force which depends upon the product of the two interacting sources.
This led Einstein to start with the electromagnetic force and gravity as the first attempt to demonstrate the unification
of the fundamental forces. It turns out that this was the wrong place
to start, and that gravity will be the last of the forces to unify with
the other three forces. Electroweak unification
(unification of the electromagnetic and weak forces) was demonstrated
in 1983, a result which could not be anticipated in the time of
Einstein's search. It now appears that the common form of the gravity
and electromagnetic forces arises from the fact that each of them
involves an exchange particle of zero mass, not because of an inherent
symmetry which would make them easy to unify.
Feynman Diagrams
Feynman diagrams are graphical ways to represent exchange forces. Each point at which lines come together is called a vertex, and at each vertex one may examine the conservation laws which govern particle interactions. Each vertex must conserve charge, baryon number and lepton number.
Developed by Feynman to decribe the
interactions in quantum electrodynamics (QED), the diagrams have found
use in describing a variety of particle interactions. They are spacetime
diagrams, ct vs x. The time axis points upward and the space axis to
the right. (Particle physicists often reverse that orientation.)
Particles are represented by lines with arrows to denote the direction
of their travel, with antiparticles having their arrows reversed.
Virtual particles are represented by wavy or broken lines and have no
arrows. All electromagnetic interactions can be described with
combinations of primitive diagrams like this one.
Only
lines entering or leaving the diagram represent observable particles.
Here two electrons enter, exchange a photon, and then exit. The time and
space axes are usually not indicated. The vertical direction indicates
the progress of time upward, but the horizontal spacing does not give
the distance between the particles.
Other electromagnetic process can be represented, as in the examples
below. A backward arrow represents the antiparticle, in these cases a
positron. Keep in mind that time progresses upward, and that a downward
arrow is not a particle progressing downward, but an antiparticle
progressing upward ( forward in time).
After being introduced for electromagnetic processes, Feynman
diagrams were developed for the weak and strong interactions as well.
Forms of primitive vertices for these three interactions are
Particle interactions can be represented by diagrams with at least
two vertices. They can be drawn for protons, neutrons, etc. even though
they are composite objects and the interaction can be visualized as
being between their constituent quarks.
lll -Exchange Particles- lll
Photon: Feynman Diagrams and the electromagnetic force Photon is the name given to a quantum of light or other electromagnetic radiation. The photon energy is given in the Planck relationship. The photon is the exchange particle responsible for the electromagnetic force. The force between two electrons can be visualized in terms of a Feynman diagram as shown below. The infinite range of the electromagnetic force is owed to the zero rest mass of the photon. While the photon has zero rest mass, it has finite momentum, exhibits deflection by a gravity field, and can exert a force.
The photon has an intrinsic angular momentum or "spin" of 1, so that the electron transitions which emit a photon must result in a net change of 1 in the angular momentum of the system. This is one of the "selection rules" for electron transitions.
Gluons: Feynman Diagrams and the Strong Force
At the most fundamental level, the strong force is an exchange force between quarks mediated by gluons. The use of Feynman diagrams
to visualize the strong interaction involves primitive vertices with
quarks and gluons. Quarks interact with each other by the exchange of
gluons; a primitive vertex in the Feynman diagram involves a change in "
color" and can take the form
The gluons carry the "color charge" and therefore the emergent quark
will not have the same color as the entering quark. This process is very
different from the electromagnetic force since the photon as the
exchange particle for the force between charges does not itself carry
charge. An interaction between quarks could be represented by the
diagram
The interaction depicted here is responsible for binding quarks together into mesons and baryons, and responsible for holding protons and neutrons together to form nuclei.
Particles can decay via the strong interaction, and if such a decay
pathway is available to a particle, it decays very quickly - on the
order of 10-23 seconds. An example is the decay
Δ0 → p+ + π-. Note that the delta baryon Δ0
has the same quark makeup as the neutron, but its mass is much larger.
Its mass is sufficient for this decay to be energetically favorable.
Because gluons carry color, the property associated with the strong
interaction, then they can interact with each other. Possible Feynman
diagrams for those interactions are
This possibility of "glueballs" greatly increases the complexity of
the strong interaction. In addition to bound quarks, there can be
collections of bound gluons hanging around.
Intermediate Vector Bosons (W and Z): Feynman Diagrams and the Weak Force
The W and Z particles are the massive exchange particles which are involved in the nuclear weak interaction, the weak force between electrons and neutrinos. They were predicted by Weinberg, Salam, and Glashow in 1979 and measured at CERN
in 1982. The prediction included a prediction of the masses of these
particles as a part of the unified theory of the electromagnetic and
weak forces, the electroweak unification.
"If the weak and electromagnetic forces are essentially the same, then
they must also have the same strength. The fact that the experimentally
observed strengths seem quite different is attributed to the masses of
the W and Z particles-under certain conditions a force of large strength
can have the appearance of a force of small strength if the particle
that carries the force is very massive. Theoretical calculations show
that at a fundamental level the weak and electromagnetic forces have the
same strength if the W and Z particles have masses of 80 and 90 GeV
respectively." The masses measured at CERN were 82 and 93 GeV, a
brilliant confirmation of the electroweak unification.
The experiments at CERN detected a total of 10 W bosons and 4 Z bosons. In the extended experiment at Fermilab's Tevatron known as "Run 1" (1992-96), the D0 detector facility
measured over 100,000 W particles. The D0 value for the mass of the W
is 80.482 +/- 0.091 GeV. Current values combining the experiments at the
Tevatron and at CERN's LEP electron-positron collider are MW = 80.41 +/- 0.18 GeV and MZ = 91.1884 +/- 0.0022 GeV.
A free neutron will decay by emitting a W-, which produces an electron and an antineutrino.
When a neutrino interacts with a neutron, a W- can be exchanged, transforming the neutron into a proton and producing an electron.
This interaction is the same as the one at left since a W+ going right to left is equivalent to a W- going left to right.
A neutron or proton can interact with a neutrino or antineutrino by the exchange of a Z0.
One of the four fundamental forces, the weak interaction involves the exchange of the intermediate vector bosons, the W and the Z. Since the mass of these particles is on
the order of 80 GeV, the uncertainty principle dictates a range of about 10-18 meters which is about .1% of the diameter of a proton.
The weak interaction changes one flavor of quark into another. For example, in the neutron decay depicted by the Feynman diagram at left above, one down quark is changed to an up quark, transforming the neutron into a proton.
The primitive vertices in the Feynman diagrams for the weak
interaction are of two types, charged and neutral. For leptons they take
the following form
The electron is used as an example in these diagrams, but any lepton
can be substituted on the incoming side. The exit side (top) will be the
same for the neutral vertex, but determined by the charge of the W in
the charged vertex. Besides conserving charge, the vertex must conserve lepton number, so the process with the electron can produce an electron neutrino but not a muon neutrino.
The neutral interaction is simpler to conceive,
but rarely observed because it competes with the much stronger
electromagnetic interaction and is masked by it.
With the charged vertices, one can postulate an interaction like m, ne -> e, nm
and draw a Feynman diagram for it. This interaction is not likely to be
oberved because of the incredible difficulty of observing the
scattering of neutrinos, but it suggests other interactions which may be
obtained by rotating or twisting the diagram.
With a twist of the Feynman diagram above, one can arrive at the interaction responsible for the decay of the muon,
so the structures obtained from the primitive vertices can be used to
build up a family of interactions. The transformation between the two
Feynman diagrams can also be seen as an example of crossing symmetry.
The charged vertices in the weak interaction with quarks take the form
So it is seen that the quark changes its flavor when interacting via the W- or W+. As drawn, this interaction cannot be observed because it implies the isolation of an up quark. Because of quark confinement,
isolated quarks are not observed. But rotating the Feynman diagram
gives an alternative interaction, shown below for both electron and muon
products.
This suggests the weak interaction mechanism for the decay of the pion, which is observed to happen by the muon pathway.
The weak interaction in the electron form at left above is responsible for the decay of the neutron and for beta decay in general.
The graviton is the exchange particle for the gravity force.
Although it has not been directly observed, a number of its properties
can be implied from the nature of the force. Since gravity is an inverse square force of apparently infinite range, it can be implied that the rest mass of the graviton is zero.
The Higgs boson is a hypothetical massive elementary particle that is predicted to exist by the Standard Model (SM) of particle physics. Its existence is predicted by the Standard Model to explain how spontaneous breaking of electroweak symmetry (the Higgs mechanism) takes place in nature, which in turn explains why other elementary particles have mass.
Its discovery would further validate the Standard Model as essentially
correct, as it is the only elementary particle predicted by the Standard
Model that has not yet been observed in particle physics experiments. If shown to exist, it is expected to be a scalar boson. (Bosons are particles with integer spin,
and scalar bosons have spin 0.) Alternative sources of the Higgs
mechanism that do not need the Higgs boson are also possible and would
be considered if the existence of the Higgs boson were ruled out. They
are known as Higgsless models.
Experiments to find out whether or not the Higgs boson exists are currently being performed using the Large Hadron Collider (LHC) at CERN, and were performed at Fermilab's Tevatron
until its closure in late 2011. Some theories suggest that any
mechanism capable of generating the masses of elementary particles must
become visible at energies above 1.4 TeV; therefore, the LHC (colliding two 3.5 TeV beams) is expected to be able
to answer the question whether or not it actually exists. In December 2011, the two main experiments at the LHC (ATLAS and CMS) both reported independently that their data hints at a possibility the Higgs may exist with a mass around 125 GeV/c2 (about 133 proton masses, on the order of 10-25 kg).
It is also believed that the original range under investigation has
been narrowed down considerably and that a mass outside approximately
115–130 GeV/c2 is very likely to be ruled out.
No conclusive answer yet exists, although it is expected that the LHC
will provide sufficient data by the end of 2012 for a definite answer
one way or the other.
In the popular media, the particle is sometimes referred to as the God particle, a title generally disliked by the scientific community as a media hyperbole that misleads readers
All the known forces in the universe are manifestations of four fundamental forces, the strong, electromagnetic, weak, and gravitational
forces. But why four? Why not just one master force? Those who joined
the quest for a single unified master force declared that the first step
toward unification had been achieved with the discovery of the
discovery of the W and Z particles,
the intermediate vector bosons,
in 1983. This brought experimental verification of particles whose
prediction
had already contributed to the Nobel prize awarded to Weinberg, Salam,
and
Glashow in 1979. Combining the weak and electromagnetic forces into a
unified "electroweak" force, these great advances in both theory and
experiment provide encouragement for moving on to the next step, the "grand unification" necessary to include the strong interaction.
While electroweak unification was hailed as a great step forward,
there remained a major conceptual problem. If the weak and
electromagnetic forces are part of the same electroweak force, why is it
that the exchange particle for the electromagnetic interaction, the
photon, is massless while the W and Z have masses more than 80 times
that of a proton! The electromagnetic and weak forces certainly do not
look the same in the present low temperature universe, so there must
have been some kind of spontaneous symmetry breaking
as the hot universe cooled enough that particle energies dropped below
100 GeV. The theories attribute the symmetry-breaking to a field called
the Higgs field, and it requires a new boson, the Higgs boson, to
mediate it.
Illustration courtesy Fermilab,
D0 Experiment.
Early formulation of the theories estimated that the Higgs boson
would have mass energy in excess of 1 TeV, making the energies for
discovery almost unattainable on the earth. Now, since the discovery of
the top quark,
there is tantalizing evidence that the Higgs boson may have energies in
the range of a few hundred GeV and therefore within the range of
present day accelerators. At Fermilab, data from the D0 detector
facility is used with the masses of the W and the T quark to estimate
the mass of the Higgs boson. Suggestions that it may have a mass below
200 GeV have made it one of the high priorities for high energy physics.
Searching for the Higgs boson is one of the high priority objectives of the Large Hadron Collider
at CERN. At the end of 2011, the LHC results appear to limit the Higgs
to between 114 and 145 GeV if it is to fit in the standard model of
particle physics.
Grand unification refers to unifying the strong interaction with the unified electroweak
interaction. The basic problem of "restoring the broken symmetry"
between the strong and electroweak forces is that the strong force works
only on colored particles and the leptons don't have color. You have to be able to convert quarks to leptons
and vice versa. But this violates the conservation of baryon number,
which is a strong experimental nuclear physics principle.
Baryon number minus lepton number (B-L) would still be conserved as
a quark is changed to an anti-lepton. The required mass of the
exchange boson is 1015 eV, which is more like the mass of a visible dust particle than that of a nuclear entity. This particle is called the X-boson.
One prediction of the grand unified theories is that the proton is unstable at some level.
In the 1970's, Sheldon Glashow and Howard Georgi proposed the grand
unification of the strong, weak, and electromagnetic forces at energies
above 1014 GeV. If the ordinary concept of thermal energy applied at such times, it would require a temperature of 1027 K for the average particle energy to be 1014 GeV.
The unification of the strong force is well beyond our reach at the present time, and the unification of gravity
with the other three is out of reach for earthbound experiments. This
has led to greater cooperation between high-energy particle physicists
and astrophysicists as each group realizes that some of their answers
can only come from the other.
One of the surprises of modern science is that atoms and sub-atomic
particles do not behave like anything we
see in the everyday world. They are not small balls that bounce around;
they have wave properties.
The Standard Model theory can mathematically describe all the
characteristics and interactions that we see for these particles, but
our everyday intuition will not help us on that tiny scale.
Physicists use the word "quantum," which means "broken into increments or parcels," to describe the physics of
very small particles. This is because certain properties only take on discrete values.
For example, you can only find electric charges that are an
integer multiples of the electron's charge (or 1/3 and 2/3 for quarks).
While quarks have a fractional electric charge of 2/3 and 1/3
electron charges, they are only found in composite particles that have
an integral electric charge. You can never observe an isolated quark.
A few of the important quantum numbers of particles are:
Electric charge. Quarks may have 2/3 or 1/3 electron charges, but they only form composite particles with integer
electric charge. All particles
other than quarks
have integer multiples of the electron's charge.
Color charge. A quark carries one of three color charges and a gluon carries one of eight color-anticolor charges.
All other particles are color neutral.
Flavor. Flavor distinguishes quarks
(and leptons) from one another.
Spin. Spin is a bizarre but important physical quantity. Large objects like planets or marbles
may have angular
momentum and a magnetic field because they spin. Since particles also to appear to have their own
angular momentum and tiny magnetic moments, physicists called this particle property spin. This is a misleading
term since particles are not actually "spinning." Spin is quantized to units of 0, 1/2, 1, 3/2
(times
Planck's Constant, ) and so on.
Spin is the internal angular momentum of a
particle, in units of.
= 1.055 x 10-34 J s. This is
Planck's Constant.
mmmmmmmmmmmm
The Standard Model - What holds it together?
- The Pauli Exclusion Principle -
We can use these quantum particle properties to categorize the particles we find.
At one time, physicists thought that no two particles in the same quantum state could exist in the
same place at the same time. This is called the Pauli Exclusion Principle, and it explains
why there is chemistry.
But it has been since discovered that a certain group of particles do not obey this principle.
Particles that do obey the Pauli Exclusion Principle are called fermions, and those that
do not are called bosons.
Imagine there is a large family of identical fermion siblings spending the night at the Fermion Motel,
and there is another large family of identical boson siblings spending the night at the Boson Inn.
Fermions behave like squabbling siblings, and not only refuse to share a room but also insist on
rooms as far as possible from each other. On the other hand, boson siblings prefer to share the same room.
(Since fermions rent more rooms than bosons, motel owners prefer doing business with fermions. Some motels
even refuse to rent rooms to bosons!)
For example, no two electrons in a single atom can have the same four quantum numbers; if n, l, and ml are the same, ms must be different such that the electrons have opposite spins, and so on.
Integer spin particles, bosons,
are not subject to the Pauli exclusion principle: any number of
identical bosons can occupy the same quantum state, as with, for
instance, photons produced by a laser and Bose-Einstein condensate.
The three types of particles from which the ordinary atom is made—protons, electrons, and neutrons—are
all subject to it, and the structure and chemical behavior of atoms is
due to it. It causes atoms to take up the space they do, since electrons
cannot all congregate in the lowest-energy state but must occupy higher
energy states at a distance from lower-energy electrons, therefore
matter made of atoms occupies space rather than being condensed. As
such, the Pauli exclusion principle underpins many properties of
everyday matter, from its large-scale stability to the periodic table of the elements.
mmmmmmmmm
- Fermions and bosons -
FERMIONS
A fermion is any particle that has an odd half-integer (like 1/2, 3/2, and so forth) spin.
Quarks and leptons, as well as most composite particles, like protons and neutrons, are fermions.
For reasons we do not fully understand, a consequence of the odd half-integer spin is that fermions obey
the Pauli Exclusion Principle and therefore cannot co-exist in the same state at same location at the same time
BOSONS
Bosons are those particles which have an integer spin (0, 1, 2...).
All the force carrier particles are bosons, as are
those composite particles with an even number of fermion particles
(like mesons).
* The predicted graviton has a spin of 2.
The nucleus of an atom is a fermion or boson depending on whether the total number of its protons
and neutrons is odd or even, respectively.
Recently, physicists have discovered that this has caused some very
strange behavior in certain atoms under unusual conditions, such as
very cold helium.
Helium has a boson nucleus (two neutrons and two protons), so it does not
ever crystallize, even when cooled to almost absolute zero.
It becomes a "superfluid," which is a liquid with strange properties
such as having zero viscosity and no surface tension.
We will probably discover other strange properties of atoms with boson nuclei in the future.
All known elementary and composite particles are bosons or fermions, depending on their spin: particles with half-integer spin are fermions; particles with integer spin are bosons. In the framework of nonrelativistic quantum mechanics, this is a purely empirical observation. However, in relativisticquantum field theory, the spin-statistics theorem shows that half-integer spin particles cannot be bosons and integer spin particles cannot be fermions
Fermions (Wikipedia)
By definition, fermions are particles which obey Fermi–Dirac statistics: when one swaps two fermions, the wavefunction of the system changes sign.[1] This "antisymmetric wavefunction" behavior implies that fermions are subject to the Pauli exclusion principle, i.e. no two fermions can occupy the same quantum state
at the same time. This results in "rigidity" or "stiffness" of states
that include fermions (atomic nuclei, atoms, molecules, etc.), so
fermions are sometimes said to be the constituents of matter, while bosons are said to be the particles that transmit interactions (i.e. force carriers) or the constituents of electromagnetic radiation.
The quantum fields of fermions are fermionic fields, obeying canonical anticommutation relations.
The Pauli exclusion principle for fermions and the associated rigidity of matter is responsible for the stability of the electron shells
of atoms (thus for the stability of atomic matter), and for the
complexity of atoms (making it impossible for all atomic electrons to
occupy the same energy level), thus making complex chemistry possible. It is also responsible for the pressure within degenerate matter, which largely governs the equilibrium state of white dwarfs and neutron stars. On a more everyday scale, the Pauli exclusion principle is a major contributor to the Young's modulus of elastic material.
All known fermions are particles with half-integer spin: as an observer
circles a fermion (or as the fermion rotates 360° about its axis) the wavefunction of the fermion changes sign. In the framework of non-relativistic quantum mechanics, this is a purely empirical observation. However, in relativisticquantum field theory, the spin-statistics theorem shows that half-integer spin particles cannot be bosons and integer spin particles cannot be fermions
The Standard Model distinguishes two types of elementary fermions: quarks and leptons. In total, 24 different fermions are recognized, 6 quarks and 6 leptons, each with a corresponding antiparticle:
W and Z bosons are the force carriers which mediate the weak force.
Gluons are the fundamental force carriers underlying the strong force.
In addition, the standard model postulates the existence of Higgs bosons, which give other particles their mass via the Higgs mechanism.
Finally, many approaches to quantum gravity postulate a force carrier for gravity, the graviton, which is a boson of spin 2.
- Fermions and bosons
We have answered the questions,
"What is the world made of?"
and
"What holds it together?"
The world is made of six quarks and six leptons. Everything we see is a conglomeration of quarks
and leptons.
There are four fundamental forces and there are force carrier particles associated with
each force.
We have also discussed how a particle's state (set of quantum numbers) may affect how it
interacts with other particles.
These are the essential aspects of the Standard Model. It is the most complete explanation of the fundamental particles
and interactions to date.
Names and descriptions are only a small
part of any physical theory; the concepts, rather than physics vocabulary,
are the critical elements.
The Contemporary Physics Education Project has
summarized the essential aspects of the Standard Model in a single chart. This site includes an electronic version
of this chart, but you can also order your own copy from CPEP.
The Standard Model - Particle decays and annihiliations
The Standard Model - Particle decays and annihiliations - What is decay?
The Standard Model explains why some particles decay into other particles.
In nuclear decay, an atomic nucleus can
split into smaller nuclei. This makes sense: a bunch of
protons and neutrons divide into smaller bunches of protons and neutrons.
But the decay of a fundamental particle cannot mean splitting into its constituents, because
"fundamental" means it has no constituents.
Here, particle decay refers to
the transformation of a fundamental particle into other fundamental particles.
This type of decay is strange, because the end products are not pieces of the starting
particle, but totally new particles.
Nuclear Decay
Particle Decay
In this section we will discuss the types of decay, how they happen, and under what
circumstances a decay will or will not happen.
The Standard Model - Particle decays and annihiliations - Radioactivity
In the late 1800s the German physicist, Wilhelm Röntgen,
discovered a strange
new ray produced when an electron beam struck a piece of metal.
Since these were rays of an unknown nature, he called them "x rays".
Two months after this discovery, the
French physicist, Henri Becquerel,
was studying fluorescence,
when he found that photographic plates were exposed in the presence
of some ores, even when the plates were wrapped in black paper.
Becquerel realized that these materials, which included
uranium, emitted energetic rays without any energy input.
Becquerel's experiments showed that some natural process must be
responsible for certain elements releasing energetic x rays.
This suggested that
some elements were inherently unstable, because these elements would
spontaneously release different forms of energy. This release of energetic
particles due to the decay of the unstable nuclei of atoms is called
radioactivity.
The Standard Model - Particle decays and annihiliations - Radioactive particles
Scientists eventually identified several
distinct types of radiation, the particles resulting from radioactive decays.
The three types of radiation were
named after the first three letters of the Greek alphabet:
(alpha),
(beta), and
(gamma).
Alpha particles are helium nuclei (2 p, 2 n):
Beta particles are speedy electrons:
Gamma radiation is a high-energy photon:
These three forms of
radiation can be distinguished by a magnetic field since
the positively-charged alpha particles curve in one direction,
the negatively-charged beta particles curve in the opposite direction,
and the electrically-neutral gamma radiation doesn't curve at all.
Alpha particles can be stopped
by a sheet of paper,
beta particles by aluminum, and gamma radiation
by a block of lead. Gamma radiation can penetrate
very far into a material, and so it is gamma radiation that poses the
most danger when working with radioactive materials, although all types of
radiation are very dangerous.
Sadly, it took scientists many years to
realize the perils of radioactivity...
Many heavy elements decay into simpler things. But a close observation of these decays
reveals several confusing problems.
Many heavy elements decay into simpler things. But a close observation of these decays
reveals several confusing problems.
Consider uranium-238 decay.
A lump of uranium-238 will decay at a constant rate such that
in 4,460,000,000 years -- give or take a few days -- half the uranium will be gone. But there is no
way to tell when a specific uranium atom will decay; it could decay five minutes from now, or
in ten billion years. Why will an atom decay only according to some probability?
Uranium-238 has a mass of 238.0508 atomic mass units (u). It can decay into
thorium (234.0436 u) and an alpha particle (4.0026 u).
But uranium's mass minus the mass of its decay products is 0.0046 u.
Why is there missing mass?
The Standard Model - Particle decays and annihiliations - A look into the nucleus
We will answer these questions soon, but first we need to look into the nature of the
nucleus and quantum mechanics.
Protons are positive and electrically repel one another. A nucleus would blow
apart if it weren't "glued" together by the gluon particles which affect every part
of the nucleus. This is called the residual strong force.
Think of a nucleus as a tightly coiled spring
which is the electrical repulsion, held in place by a very big rope
which is the residual strong force.
Even though there is a lot of stored-up energy in the spring,
it can't release the energy because the rope is too strong.
The Standard Model - Particle decays and annihiliations - If it can happen, it will
Subatomic particles do not behave like everyday objects.
We can't really say what a particle will do, only
what a particle might do.
Particles move around like everyday objects and have momentum,
but they also have wave properties. Quantum mechanics, the mathematical basis
for our theories about particles,
explains the behavior of particles in terms of probabilities.
Since particles are wave-like, it is impossible to know both their position and their
momenta. While it is easier to think of particles as point-like spheres
(which is how
we have illustrated them throughout this site) this is misleading since
they are better thought of as fuzzy regions in which you are most
likely to find the particle.
Protons and neutrons migrate around inside a nucleus.
There is a tiny, tiny chance that a conglomeration of
two protons and two neutrons (which form an alpha particle) may, at the same instant, actually
migrate outside the nucleus.
There is a greater chance of this happening in a large
nucleus than in a small one.
The alpha particle would then be free of the residual strong force trapping it inside the nucleus,
and like a suddenly released spring,
the charged alpha particle would fly away from
the nucleus.
This idea that "if it can happen, it will happen!" is fundamental to quantum
mechanics. For some atoms there is a certain
probability that it will undergo
radioactive decay due to the possibility
that the nucleus may --for the shortest
of instants-- exist in a state that allows it to blow apart. You cannot predict when
a particular atom will decay, but you can determine the chance that it will decay
in a certain period of time.
The Standard Model - Particle decays and annihiliations - Half life
A lump of uranium left to itself will gradually decay, one nucleus at a time.
The rate of decay is measured by how
long it would take for half of a given bunch of uranium atoms to decay
(the half-life).
The decay of an individual uranium nucleus is completely unpredictable, but we can accurately
predict the way a large lump of uranium will decay.
It is upsetting to think that chance can
rule physical properties. In response to this theory
Einstein proclaimed "God doesn't play dice!" (Einstein was wrong.)
The Standard Model - Particle decays and annihiliations - Missing mass
We still need to answer the question, where does the missing mass in a radioactive decay go?
Recall that we said that when uranium decays into thorium and an alpha particle, 0.0046 u
of mass appears to have been lost.
As Einstein said,
When uranium nuclei undergo radioactive
decay, some of their mass is converted into
kinetic energy (the energy of the moving particles).
This conversion of energy is observed as a loss of mass.
The Standard Model - Particle decays and annihiliations - Particle decay mediators
While the nucleus of an atom can decay into a less massive nucleus by splitting apart,
how does a
fundamental particle decay into other fundamental particles? Fundamental particles cannot
split apart, because they have no constituents,
but rather they somehow turn into other particles.
A charm quark (c) decays into a
less massive particle (strange quark, s) and
a force carrier particle (W boson) which then
decays to u and d quarks
It turns out that when a fundamental particle decays, it
changes into a less massive particle and a
force-carrier particle (always a W boson for fundamental particle decays).
These force carriers may then re-emerge as other particles.
So, a particle does not just change into another particle type;
there is an intermediate force-carrier particle which
mediates particle decays.
In many cases, these temporary force-carrier
particles seem to violate the conservation of energy
because their mass is greater than the available energy in the reaction. However,
these particles exist so briefly that, because of
Heisenberg's Uncertainty Principle, no rules are broken.
These are called virtual particles.
In 1927, Werner Heisenberg determined that it is impossible to
measure both a particle's position and its momentum exactly.
The more precisely we determine one, the less we know about the other.
This is called the Heisenberg Uncertainty Principle, and it is a fundamental
property of quantum mechanics.
The precise relation is:
This constant is Planck's constant divided by two; Planck's constant is represented by the
symbol , or "h-bar," and equals 1.05 x 10-34
joule-seconds, or 6.58 x 10-22 MeV-seconds.
The act of measuring a particle's position will affect your knowledge of its momentum, and
vice-versa.
We can also express this principle in terms of energy and time:
This means that if a particle exists for a very brief time, you cannot precisely determine
its energy. A short-lived particle could have a tremendously uncertain energy, which leads
to the idea of virtual particles.
The Standard Model - Particle decays and annihiliations - Virtual particles
Particles decay via force carrier particles.
But in some cases a particle may decay via
a force-carrier particle with more mass then the
initial particle. The intermediate particle is immediately transformed into
lower-mass
particles. These short-lived high-mass force-carrier particles seem to violate the
laws of conservation of energy and mass -- their mass just can't come out of nowhere!
A result of the Heisenberg
Uncertainty principle is that these high-mass particles may come into being if they
are incredibly short-lived. In a sense, they escape reality's notice.
Such particles are called virtual particles.
Virtual particles do not violate the conservation of energy.
The kinetic energy plus mass of the initial decaying particle and the
final decay products is equal. The virtual particles
exist for such a short time that they can never be observed.
Most particle processes are
mediated by virtual-carrier particles. Examples include
neutron beta decay,
the production of charm particles, and the
decay of an eta-c particle, all of which we will explore in depth soon.
The Standard Model - Particle decays and annihiliations - Different interactions
Strong, electromagnetic, and weak interactions all cause particle decays.
However, only weak interactions can cause the decay of fundamental particles.
Weak Decays:
Only weak interactions can
change a fundamental particle into another type of particle.
Physicists call particle types "flavors."
The weak interaction can change a charm quark into a strange quark
while emitting a virtual W boson (charm and strange are flavors).
Only the weak interaction (via the W boson) can
change flavor and allow the decay of a truly fundamental particle.
Electromagnetic Decays:
The 0 (neutral pion)
is a meson. The quark and antiquark can
annihilate; from the annihilation come two photons. This
is an example of an electromagnetic decay.
Strong Decays:
The particle is ameson. It can undergo a strong decay into two gluons (which emerge as
hadrons)
The strong
force-carrier particle, the gluon, mediates decays involving color changes.
The weak force-carrier particles, W+ and
W-, mediate decays in which particles change flavor (and
electric charge)
The Standard Model - Particle decays and annihiliations - Annhiliations
Annihilations are of course not decays, but they too occur via virtual particles.
In an annihilation a matter
and an antimatter particle
completely annihilate into energy.
That is, they interact with each
other, converting the energy of their
previous existence into a very energetic
force carrier particle (a gluon, W/Z, or photon).
These force carriers, in turn,
are transformed into other particles.Quite often, physicists will annihilate two
particles at tremendous energies in
order to create new, massive particles.
The Standard Model - Particle decays and annihiliations - Bubble chamber and decays
This is an actual bubble chamber photograph of an antiproton
(entering from the bottom of the picture),
colliding with a proton (at rest), and
annihilating.
Eight pions were produced in this annihilation.
One decayed into a + and a .
The paths of positive and negative pions curve opposite ways in the
magnetic field, and the neutral
leaves no track.
Bubble chambers are an older type of detector. As charged particles pass through
a bubble chamber, they leave a trail of tiny bubbles that make it easy to track the
particles.
We have talked a lot about decays and annihilations, so let's now look at some
examples of these processes.
The Standard Model - Particle decays and annihiliations - Neutron beta decays
A neutron (udd) decays to a proton (uud),
an electron,
and an antineutrino. This is called
neutron beta decay.
(The term beta ray was used for electrons in nuclear decays
because they didn't know they were electrons!)
Frame 1: The neutron (charge = 0) made of up, down, down quarks.
Frame 2: One of the down quarks is transformed into an up quark.
Since the down quark has a charge of -1/3 and and the up quark has a charge
of 2/3, it follows that this process is mediated by a virtual W- particle, which carries away a (-1)
charge (thus charge is conserved!)
Frame 3: The new up quark rebounds away
from the emitted W-. The neutron now has become a proton.
Frame 4: An electron and antineutrino emerge from the virtual W-
boson.
Frame 5: The proton, electron, and the antineutrino move away from
one another.
The intermediate stages of this process occur in
about a billionth of
a billionth of a billionth of a second, and are not observable.
The Standard Model - Particle decays and annihiliations - Electron / positron annhiliation
When an electron and positron (antielectron)
collide at high energy, they can annihilate to produce
charm quarks which then produce
D+ and D- mesons.
Frame 1: The electron and positron zoom towards their certain doom.
Frame 2: They collide and annihilate,
releasing tremendous amounts of energy.
Frame 3: The electron and positron
have annihilated into a photon, or a
Z particle, both of which may be
virtual force carrier particles.
Frame 4: A charm quark and a charm
antiquark emerge from the virtual force carrier particle.
Frame 5: They begin moving apart, stretching the
color force field (gluon field) between them.
Frame 6: The quarks move apart, further spreading their force field.
Frame 7: The energy in the
force field increases with the separation between the quarks.
When there is sufficient energy in the
force field, the energy is converted into a
quark and an anti-quark
(remember ).
Frames 8-10: The quarks separate into
distinct, color-neutral particles: the
D+
(a charm and anti-down quark) and
D-
(an anti-charm and down quark) mesons.
The intermediate stages
of this process occur in about a
billionth of a billionth of a billionth of a second, and are not observable.
The Standard Model - Particle decays and annihiliations - Top production
A quark (from within a proton) and an antiquark
(from an antiproton) colliding at
high energy can annihilate to produce a top
quark and a top antiquark, which then decay into other particles.
Frame 1: One of the proton's quarks and one of the
antiproton's antiquarks are heading toward a collision.
Frame 2: The quark and antiquark collide and annihilate....
Frame 3: ...into virtual gluons.
Frame 4: A top and antitop quark emerge from the gluon cloud.
Frame 5: These quarks begin moving apart, stretching the
color force field
(gluon field) between them.
Frame 6: Before the top quark and
antiquark have moved very far, they decay into a bottom and antibottom
quark (respectively) with the emission of W force carrier particles.
Frame 7: The new bottom quark and antibottom
quark rebound away from the emitted W force carrier particles.
Frame 8: An electron and neutrino emerge from the
virtual W- boson, and an up quark
and down antiquark emerge from the
virtual W+ boson.
Frame 9: The bottom quark and bottom antiquark,
electron, neutrino, up quark,
and down antiquark all move away from one another.
What is wrong with this picture? [We ignored the color force field that develops as the b quark and b antiquark
move apart. This energy is converted into another quark/antiquark pair; eventually only distinct,
color-neutral particles emerge (B mesons). The same is true for the u quark and d antiquark.
To see what really happens look at an analogous process in the picture of e+ and e- --> D+ and D-.]
The intermediate stages of this
process occur in about a billionth of
a billionth of a billionth of a second, and are not observable.
Unsolved Mysteries
Unsolved Mysteries - Beyond the standard model
The Standard Model answers many of the questions about the structure and stability of
matter with its six types of quarks, six types of leptons, and four forces.
But the Standard Model is not complete; there are still many unanswered questions.
Why do we observe matter and almost no antimatter if we believe there is a
symmetry between the two in the universe? What is this "dark matter" that we can't see
that has visible gravitational effects in the
cosmos? Why can't the Standard Model predict a particle's mass? Are quarks and leptons actually fundamental, or made up of even more fundamental particles?
Why are there exactly three generations of quarks and leptons?How does gravity fit into all of this?
Unsolved Mysteries - The standard model as a theory
While the Standard Model
provides a very good description of
phenomena observed by experiments, it is still an incomplete theory.
The problem is that the Standard Model cannot explain
why some particles exist as they do. For example, even though
physicists knew the masses of all the quarks except for top quark for many
years, they were simply unable to accurately predict the top quark's mass without
experimental evidence because the Standard Model lacks any explanation
for a possible pattern for particle masses.
Does this mean that the Standard Model is wrong?
No -- but we need to go beyond the Standard Model in the same way that
Einstein's Theory of Relativity extended Newton's laws of mechanics.
Isaac Newton's laws of mechanics are not wrong,
per se, but his theory only works as long as velocity is much smaller than the speed of
light. Einstein expanded
Newtonian physics with his Theory of Relativity,
which allows for the possibility of very high
velocities. We will need to extend the Standard
Model with something totally new in order to thoroughly explain
mass, gravity and other phenomena.
I wonder what we'll decide to call the new theory that replaces
the Standard Model!
The Super-Standard Model?
The New Revised Standard Model?
The Very Standard Model?
Unsolved Mysteries - Three generations
There are three "sets" of quark pairs and lepton pairs.
Each "set" of these particles is called a
generation, or family.
The up/down quarks are first generation quarks, while the electron/electron neutrino leptons are
first generation leptons.
Why are there exactly three generations of matter?
The generations increase in mass and higher generation particles
tend to decay into lower generation particles.
In the every-day world we observe only the first-generation particles
(electrons and up/down quarks). We do not know why the natural world
"needs" the two other generations, and we do not know why there are exactly
three generations
in total.
Unsolved Mysteries - What about masses?
The Standard Model cannot explain why a particle has a certain mass.
For example, both the photon and the W particle are force carrier particles:
why is the photon massless and the W particle massive?
Physicists have theorized the existence of the so-called
Higgs field, which in theory interacts with other particles to
give them mass. The Higgs field requires a particle, the
Higgs boson. The Higgs boson has not been observed,
but physicists are looking for it with great enthusiasm.
Unsolved Mysteries - Grand Unified Theory
Today, one of the major goals of particle physics is to unify
the various fundamental forces in a Grand Unified Theory
which could offer a more elegant understanding of the
organization of the universe. Such a simplification of the
Standard Model might well help to answer our questions
and point toward future areas of study.
James Maxwell took a big step toward this goal when he
unified electricity and magnetism, and physicists now understand
that at high energies the electromagnetic and
weak forces are aspects of the same force.
For years, physicists have sought for and found unified theories.
1861-1865
James Maxwell, in a series of pages,
described the interrelation of electric
and magnetic fields thereby unifying them into electromagnetism.
This led to the now-famous Maxwell's Equations.
1881-1884
Hertz demonstrated radio
waves and established that radio waves and light are
electromagnetic waves of different frequencies,
as predicted by Maxwell's theory.
1967-1970
Glashow, Salam, and Weinberg proposed a theory that unifies
electromagnetic and weak interactions.
They predicted the mass of the
W boson which mediates weak processes such as beta
decay and predicted a new type of
weak interaction and its mediating particle the Z
boson. Evidence for this new type of process was soon found.
They also predicted the Higgs Boson.
1979
The Nobel Prize was awarded to Glashow, Salam, and
Weinberg for their role
in the development of the electroweak theory,
four years before the discovery of
the W and Z bosons!
1983
The W and Z bosons were finally discovered in 1983 by the UA-1 and
UA-2 experiments at CERN.
These discoveries dramatically confirmed the Standard Model.
Detectors at today's accelerators have observed over 100,000 W's and millions of Z's.
In his later years Einstein attempted, but failed, to
write a theory which united gravity and electricity.
Unsolved Mysteries - Forces and the Grand Unified Theory
Physicists hope that a Grand Unified Theory will unify
the strong, weak, and electromagnetic interactions.
There have been several proposed Unified Theories, but
we need data to pick which, if any, of these theories describes
nature.
If a Grand Unification of all the interactions is possible,
then all the interactions we observe are all different aspects of the same,
unified interaction. However,
how can this be the case if strong and weak
and electromagnetic interactions are so
different in strength and effect?
Strangely enough, current data and theory
suggests that these varied
forces merge into one force when the
particles being affected are at a high enough energy.
Current work on GUT suggests the existence of another force-carrier
particle that causes the proton to decay. Such decays are extremely rare;
a proton's lifetime is more than 1032 years.
Unsolved Mysteries - Supersymmetry
Some physicists attempting
to unify gravity with the other fundamental
forces have come to a startling prediction: every
fundamental matter particle should have a massive
"shadow" force carrier particle, and every force carrier should
have a massive "shadow" matter particle. This relationship
between matter particles and force carriers is called
supersymmetry.
For example, for every type of quark there may be a type of particle called a
"squark."
No supersymmetric particle has yet been found, but experiments
are underway at CERN and
Fermilab to detect supersymmetric partner particles.
Unsolved Mysteries - String theory
Modern physics has good theories for quantum mechanics, relativity,
and gravity. But these theories do not quite work with each other.
There are problems caused by our living in three spatial dimensions.
If we lived in more than three dimensions, these problems would
naturally resolve themselves.
String Theory, one of the recent proposals of modern physics,
suggests that in a world with three ordinary dimensions and
some additional very "small" dimensions, particles are
strings and membranes. Yes, membranes in extra dimensions are weird
and hard to visualize. And what are "small dimensions?"
Unsolved Mysteries - Extra dimensions
String theory and other new proposals require more than three
space dimensions. These extra dimensions could be very small, which
is why we don't see them.
How can there be extra, smaller dimensions?
Think about an acrobat and a flea on a tight rope. The acrobat can
move forward and backward along the rope. But the flea can move
forward and backward as well as side to side. If the flea keeps
walking to one side, it goes around the rope and winds up where it
started. So the acrobat has one dimension, and the flea has two
dimensions, but one of these dimensions is a small closed loop.
So the acrobat cannot detect any more than the one dimension of the rope, just as we
can only see the world in three dimensions, even though it might well have many more. This
is impossible to visualize, precisely because we can only visualize things in three
dimensions!
Unsolved Mysteries - Dark matter
Ready for a mind-boggling idea?
The majority
of the universe may not be made of the same type of matter as the
Earth. We infer from
gravitational effects the presence of this dark
matter, a type of matter that we cannot see. There is strong evidence
that it might not be made up of protons, neutrons, and electrons.
What is dark matter, then? We don't know. Perhaps it is composed of
neutrinos, or even more exotic forms of matter, like neutralinos, one
of the theoretical supersymmetric particles.
Accelerators and Particle Detectors
Accelerators and particle detectors - How do we know any of this? - Testing a theory
We have discussed various aspects of the Standard Model and have delved deep into the
world of tiny, invisible subatomic particles with strange names. All of this scientific
theory may start to look like magic, but it's important to realize that physicists do not
just sit around and make up this stuff. They test their hypotheses, and create new
theories from the results of their experiments.
To test theories, physicists put together experiments and use what
they already know to find out what they do not know. These
experiments may be simple, or they may be huge and complicated.
The Standard Model rises out of thousands of years of scientific inquiry, but most of the
experiments that have given rise to our current conception of particle physics have occurred
relatively recently. The story of how physicists experiment to test and create
theories in modern particle physics is one which starts less than a hundred years ago...
Accelerators and particle detectors - How do we know any of this? - Searching for the atom's structure
In 1909, the prevailing theory of the atom's structure was that atoms were mushy, semi-
permeable balls, with bits of charge strewn around them. This theory worked just fine
for most experiments about the physical world.
Physics, however, is not only interested in how the world appears to
operate, but how it actually works. And so in 1909 a man named
Ernest Rutherford set up an experiment to test the validity of the prevailing
theory. In doing so he established a way that for the first time physicists could "look into"
tiny particles they couldn't see with microscopes.
In Rutherford's experiment, a radioactive source shot a stream of alpha particles at
a sheet of very thin gold foil which stood in front of a screen. The alpha
particles would make little flashes of light where they hit the
screen.
The alpha particles were expected to pass right through the very thin gold foil and make
their marks in a small cluster on the screen.
Accelerators and particle detectors - How do we know any of this? - Rutherford's Result
If atoms were permeable, neutral balls, then the alpha particles should simply pass
through the gold foil and strike the back of the screen. But much to everyone's surprise,
some of the alpha particles were deflected at large angles to the foil; some even hit the screen
in front of the foil! Obviously some other explanation was needed.
Accelerators and particle detectors - How do we know any of this? - Rutherford's analysis
Since some of the positive alpha particles were
substantially deflected, Rutherford concluded that there must
be something inside an atom for the alpha
particles to bounce off of that is small, dense,
and positively charged: the nucleus.
Accelerators and particle detectors - How do we know any of this? - How physicists experiment
Rutherford's experiment set the tone for the realm of experimentation in particle physics;
in fact, almost all particle physics experiments today use the same basic elements that
Rutherford did:
A beam (in this case, the alpha particles)
A target (the gold atoms in the foil)
A detector (the zinc sulfide screen)
In addition, Rutherford established the practice of "seeing" into the sub-atomic realm by
using particle beams, and particle physicists today follows his experimental lead by
inferring the actual nature of particles and interactions from the
frequently counterintuitive results they find.
Accelerators and particle detectors - How do we know any of this? - Deflected probe
Try it yourself!
In the following pictures, there is a target hidden by a
black cloud. To figure out the shape of the target, we shot some beams into
the cloud and recorded where the beams came out. Can you figure out the
shape of the target? Click on "Look At Answers" button to compare your
guess with the real answer.
Accelerators and particle detectors - How do we detect what's happening? - Detecting the world
Let's look at the most familiar
example of this source/target/detection scheme:
the way in which we perceive the world.
What we think of as "light" is really made up of
billions and trillions of particles called "photons."
Photons, like all particles, also have
wave characteristics.
For this reason, a
photon carries information about the
physical world because it interacts with what it hit.
For example, imagine that there is a light
bulb behind you, and a tennis ball in front of you.
Photons travel from the light bulb (source),
bounce off the tennis ball (target), and
when these photons hit your eye (detector),
you infer from the direction the photons
came from that there is a round object in front of you.
Moreover, you can tell by the different photon
wavelengths that the object is green and tan.
Our brain analyzes the information, and creates the sense of a "tennis ball"
in our mind. Our mental
model of the tennis ball helps to describe the reality around us.
We use the information of bounced-around
light waves to perceive our world.
Other animals, like dolphins and bats, emit and detect
sound waves. In fact, any kind of
reflected wave can be used to get information about
the surroundings.
Accelerators and particle detectors - How do we detect what's happening? - A better microscope
The problem with using waves to detect the
physical world is that the quality
of your image is limited by the wavelength you use
Our eyes are attuned to visible light,
which has wavelengths in the neighborhood of 0.0000005 meters.
That's small enough that we usually don't
need to worry about the wavelength-resolution problem since
we don't look at things that are 0.0000005 meters wide.
However, the wavelength of visible light is too wide to analyze
anything smaller than a cell. To observe things under higher
magnification, you must use waves with smaller wavelengths.
That's why people turn to scanning electron
microscopes when studying sub-microscopic things like viruses.
However, even the best scanning electron microscope can only show a
fuzzy picture of an atom.
Accelerators and particle detectors - How do we detect what's happening? - Wavelength - The cave
Pretend that you are unlucky enough to
fall into a cave without a flashlight.
However, you are lucky enough to have a bucket of glow-in-the-dark
basketballs. Suddenly, you hear a snuffling sound.
Is it a blood-thirsty bear, or
merely your friends playing a practical joke on you?
To find out, you desperately toss the basketballs in the direction of the
snuffling sound, and memorize where the basketballs hit. Thus, you
rapidly figure out the following outline of the being in front of you:
Yikes! Since your basketballs are so big, when they bounce off the
thing in front of you, all you can
learn about its shape is that it is wide and tall.
Fortunately, you ALSO brought a bag of glow-in-the-dark tennis balls. You
toss these in the direction of the snuffling, and are rewarded with the
following image:
Hmm..... not much better.
Tennis balls are still too big to figure out the shape
of the object they hit. You only have a rough idea about the thing's outline.
Aha! What luck! Your bag of glow-in-the-dark marbles should do the trick!
You toss these little balls at the being,
and note that you can figure out a pretty
clear image of the thing's shape. It seems to be big, hunched over, and have
enormous claws. A bear!
Your last, but gleeful, thought is that you
used the smallest possible probe to
get the most information about your fate.
No bears were harmed in the making of this web page!
Accelerators and particle detectors - How do we detect what's happening? - Wavelength - The moral
The morals of the preceding story are:
Don't throw things at hungry bears
To gather the most information about an object,
use the smallest possible probe
A "hit" by any of the probes only told you that a bear existed somewhere
within the diameter of the probe. Of the three probes, the marbles were the
most effective means of gathering information because a
marble "hit" told us that a bear existed somewhere within a very tiny area.
We would call the
basketball image "fuzzy"
because there is a lot of uncertainty about the bear's
real shape. As the probe size got smaller, the images became "sharper"
because you were increasingly certain about the
bear's shape. This quality of "sharpness" is called the
resolution of an image.
The large basketballs don't give us a lot of information
about the bear's outline, so we call this a "fuzzy" image.
The bear's outline
is fairly clear, so this is a "sharp" image.
Accelerators and particle detectors - How do we detect what's happening? - Wavelength and resolution explained
Things with long wavelengths are analogous to the basketball in the cave
story because neither can provide too much detail about what they hit.
Things with short wavelengths are like the marbles in that
they can provide you with fairly
detailed information about what they hit.
The shorter the probe's wavelength is,
the more information you can get about the target.
A good example of the wavelength
vs. resolution issue is a swimming pool. If you have a swimming pool
with waves which are 1 meter apart (a 1 meter wavelength) and push a stick
into the water, the pool's waves
just pass around the stick because the
1 meter wavelength means that the
pool's waves won't be affected by such a tiny target.
All particles have wave properties.
So, when using a particle as a probe,
we need to use particles with short wavelengths to get detailed information
about small things. As a rough rule of thumb, a
particle can only probe down to distances
equal to the particle's wavelength.
To probe down to smaller scales, the
probe's wavelength has to be made smaller.
This is all a very hand-wavy explanation of a very
difficult concept.
To explain it completely would involve
more math than we have space to get into.
Accelerators and particle detectors - How do we detect what's happening? - The physicists tool: The accelerator
Physicists can't use light to explore atomic and sub-atomic structures
because light's wavelength is too long. However, since ALL particles
have wave properties, physicists can use particles as their probes.
In order to see the smallest particles, physicists need a particle with
the shortest possible wavelength. However, most of the particles around
us in the natural world have fairly long wavelengths. How do physicists
decrease a particle's wavelength so that it can be used as a probe?
A
particle's momentum and its wavelength are inversely
related
High-energy physicists apply this principle
when they use particle accelerators
to increase the momentum of a probing particle,
thus decreasing its wavelength.
Steps:
Put your probing particle into an accelerator.
Give your particle lots of momentum by
speeding it up to very nearly the speed of light.
Since the particle now has a lot of momentum,
its wavelength is very short.
Slam this probing particle into the target and record what happens.
Accelerators and particle detectors - How do we detect what's happening? - Waves and particles
One of the most interesting property of waves is that when two
waves pass through each other,
their effects are added together. This is called
interference.
Imagine a light source being
blocked by a sheet of metal with two slits in it. A
few meters away there is a screen. For a given point along the screen, there
are two light waves hitting the screen (one through each hole). These two
light waves travel different distances to reach the screen,
so they interfere
with each other, creating an interference pattern.
It turns out that if you carry out a similar experiment using a particle beam
instead of a light source, you record a similar interference
pattern. This means
that all particles have wave properties.
For example, here's a real interference
pattern caused by an electrons scattering off gold foil:
It is a very strange concept that
what we think of as solid matter particles are,
in reality, wave-like because matter
particles have wavelengths, and
can interfere with each other.
Accelerators and particle detectors - How do we detect what's happening? - The world's meterstick
This image represents a meter stick measuring powers of ten. As you can see,
there are different methods to view the world corresponding to the size
of the thing you are viewing.
Accelerators and particle detectors - How do we detect what's happening? - Mass and energy
Quite often, physicists want to study massive, unstable particles that
have only a fleeting existence
(such as the very massive top quark.)
However, all that physicists
have around them in the every day world are very
low-mass particles.
How does one perform this amazing
feat of using particles with lesser mass to obtain particles of greater mass?
You know Albert Einstein's famous equation that where E is the energy,
m is the mass, and c is
the speed of light. Therefore,
Accelerators and particle detectors - How do we detect what's happening? - Energy-mass conversion
When a physicist wants to use particles with low mass to produce
particles with greater mass,
all she has to do is put the low-mass particles into
an accelerator, give them a lot of kinetic energy (speed),
and then collide them
together.
During this collision, the particle's kinetic energy is converted into
the formation of new massive particles.
It is through this process that we can
create massive unstable particles and study their properties.
It is as if you stage a head-on collision between two
strawberries and get several new strawberries,
lots of tiny acorns, a banana, a
few pears, an apple, a walnut, and a plum.
Accelerators and particle detectors - How do we experiment with tiny particles? - Accelerators
Accelerators solve two problems for physicists.
First, since all particles behave like
waves, physicists use accelerators to increase a
particle's momentum, thus
decreasing its wavelength enough that
physicists can use it to poke inside atoms.
Second, the energy of speedy particles is
used to create the massive particles that physicists want to study.
How do accelerators work?
Basically, an accelerator takes a particle,
speeds it up using electromagnetic
fields, and bashes the particle into a target
or other particles. Surrounding the collision point are
detectors that record the many pieces of the event.
Question: What is the nearest particle accelerator to you right now?
[ The computer monitor right in front of you (unless you
have an LCD monitor)!
] How do physicists get the particles they want to study?
Accelerators and particle detectors - How do we experiment with tiny particles? - How to obtain particles to accelerate
Electrons: Heating a metal causes electrons
to be ejected. A television, like a cathode ray tube, uses this
mechanism.
Protons: They can easily be obtained by ionizing hydrogen.
Antiparticles: To get
antiparticles,
first have energetic
particles hit a target. Then pairs of particles and antiparticles
will be created via
virtual photons or gluons.
Accelerators and particle detectors - How do we experiment with tiny particles? - Accelerating particles
It is fairly easy to obtain particles.
Physicists get electrons by heating metals;
they get protons by robbing hydrogen of its electron; etc.
Accelerators speed up charged particles by
creating large electric fields which attract or repel the particles.
This field is then moved down the accelerator, "pushing" the particles along.
In a linear accelerator the field is due to traveling electromagnetic
(E-M) waves. When an E-M wave hits a bunch of particles, those in the back
get the biggest boost, while those in the front get less of a boost. In this
fashion, the particles "ride" the front of the
E-M wave like a bunch of surfers.
Accelerators and particle detectors - How do we experiment with tiny particles? - Accelerating particles: Animation
The above is an animation of the following concept:
This is a test search string for google.
Accelerators and particle detectors - How do we experiment with tiny particles? - Accelerator design
There are several different ways to design these accelerators, each with its
benefits and drawbacks. Here's a quick list of the major accelerator design
choices:
Accelerators can be arranged to provide collisions of two types:
Fixed target: Shoot a particle at a fixed target.
Colliding beams: Two beams of particles are made to cross each other.
Accelerators are shaped in one of two ways:
Linacs: Linear accelerators,
in which the particle starts at one end and comes out the other.
Synchrotrons: Accelerators built in a circle,
in which the particle goes around and around and around...
Accelerators and particle detectors - How do we experiment with tiny particles? - Fixed target experiments
In a fixed-target experiment,
a charged particle such as an electron or a
proton is accelerated by an electric field and collides
with a target, which can be a solid, liquid, or gas.
A detector determines the
charge, momentum, mass, etc. of the resulting particles.
An example of this process is
Rutherford's gold foil experiment, in which the
radioactive source provided high-energy alpha particles, which collided with
the fixed target of the gold foil. The detector was the zinc sulfide screen.
Accelerators and particle detectors - How do we experiment with tiny particles? - Colliding beam experiment
In a colliding-beam experiment two beams of high-energy particles are made
to cross each other.
The advantage of this arrangement is that both beams have significant
kinetic energy,
so a collision between them is more likely to produce a higher mass particle
than would a fixed-target collision (with the one beam) at the same energy.
Since we are dealing with particles with a lot of momentum,
these particles have short wavelengths and make excellent probes.
Accelerators and particle detectors - How do we experiment with tiny particles? - A linear or circular accelerator? All accelerators are either linear or circular,
the difference being whether the
particle is shot like a bullet from a gun
(the linear accelerator) or whether the
particle is twirled in a very fast circle,
receiving a bunch of little kicks each
time around (the circular accelerator). Both types accelerate particles by
pushing them with an electric-field wave.
Linear accelerators (linacs) are used for fixed-target
experiments, as injectors to circular accelerators, or as linear colliders.
Fixed target:
Injector to a circular accelerator:
Linear collider:
The beams from a circular accelerator
(synchrotron) can be used for
colliding-beam experiments or
extracted from the ring for fixed-target experiments:
Colliding beams:
Extracted to hit a fixed target:
The particles in a circular accelerator go around in circles because large
magnets tweak the particle's path enough to keep it in the accelerator.
How do a
circular accelerator's
magnets make particles go in a circle?
Accelerators and particle detectors - How do we experiment with tiny particles? - What makes particles go in a circle?
To keep any object going in a circle,
there needs to be a constant force on that
object towards the center of the circle.
In a circular accelerator, an electric field makes the charged particle
accelerate, while large magnets provide the necessary inward
force to bend the particle's path in a circle.
(In the image to the left, the particle's velocity is represented by the
white arrow, while the inward force supplied by the
magnet is the yellow arrow.)
The presence of a magnetic field does not add or
subtract energy from the particles.
The magnetic field only bends the particles' paths along the arc
of the accelerator.
Magnets are also used to direct charged particle beams toward targets
and to "focus" the beams,
just as optical lenses focus light.
Question: If a
magnetic field makes electrons go clockwise,
in which direction does it make positrons go?
[ Counterclockwise! The same magnetic field makes positrons going in the opposite direction stay in the
same circle.
]
Accelerators and particle detectors - How do we experiment with tiny particles? - Advantages of accelerator design
The advantage of a circular accelerator over a
linear accelerator is that the
particles in a circular accelerator (synchrotron) go around many times,
getting multiple kicks of
energy each time around. Therefore, synchrotrons can provide very
high-energy particles without having to be of tremendous length. Moreover,
the fact that the particles go around many times means that there are many
chances for collisions at those places
where particle beams are made to cross.
On the other hand, linear accelerators are
much easier to build than circular
accelerators because they don't need the large magnets required to coerce
particles into going in a circle. Circular accelerators also need an
enormous radii in order to get
particles to high enough energies, so they are expensive to build.
Another thing that physicists need to
consider is that when a charged particle
is accelerated, it radiates away energy.
At high energies the radiation loss is
larger for circular acceleration than for linear acceleration.
In addition, the
radiation loss is much worse for accelerating light
electrons than for heavier protons. Electrons and anti-electrons (positrons)
can be brought to high energies only in
linear accelerators or in circular ones
with large radii.
Question: Can an object accelerate while keeping the same speed?
[ Yes: Speed is absolute change in position/time. But
velocity is speed and direction,
and acceleration is change in velocity/time.
So, a particle going in a circle maintains the
same speed yet changes direction, so it is changing velocity and therefore accelerating.
]
Accelerators and particle detectors - How do we experiment with tiny particles? - Major accelerators
We invite you to explore the basic plans of the
world's major accelerators so that
you can truly appreciate the differences in accelerator designs.
SLAC:
Stanford Linear Accelerator Center, in
California, discovered the charm quark (also
discovered at Brookhaven) and tau lepton;
ran an accelerator producing huge numbers of B mesons.
Fermilab:
Fermi National Laboratory Accelerator, in Illinois,
where the bottom and top quarks
and the tau neutrino were discovered.
CERN:
European Laboratory for Particle Physics, crossing
the Swiss-French border, where the W and Z particles were discovered.
BNL: Brookhaven National Lab, in New York,
simultaneously with SLAC discovered the charm
quark.
CESR:
Cornell Electron-Positron Storage Ring, in New York.
CESR performed detailed studies of the bottom quark.
DESY:
Deutsches Elektronen-Synchrotron, in Germany; gluons were discovered here.
KEK:
High Energy Accelerator Research Organization, in Japan, is
now running an accelerator producing huge numbers of B mesons.
IHEP:
Institute for High-Energy Physics, in the
People's Republic of China,
performs detailed studies of the tau lepton and charm quark.
Accelerators and particle detectors - How do we experiment with tiny particles? - The event
After an accelerator has pumped enough energy
into its particles, they collide
either with a target or each other. Each of these collisions is called an
event. The physicist's goal is to isolate each event,
collect data from
it, and check whether the particle processes of that event agree with the
theory they are testing.
Each event is very complicated since lots of particles are produced.
Most of these particles have lifetimes so short that they go an
extremely short distance before decaying into other particles,
and therefore leave no detectable tracks.
How can a physicist determine what
happened if she can never record the presence of several key particles?
Accelerators and particle detectors - How do we experiment with tiny particles? - Detectors
Just as Rutherford used
zinc sulfide to test for the presence of invisible alpha
particles and used this knowledge to determine the path of alpha particles,
modern physicists must look at particles'
decay products, and from these deduce the particles' existence.
To look for these various particles and decay products,
physicists have designed multi-component detectors that test
different aspects of an event. Each component of a
modern detector is used for measuring
particle energies and momenta, and/or
distinguishing different particle types.
When all these components work together to detect an event,
individual particles can be singled out from the multitudes for analysis.
Following each event, computers collect and interpret the
vast quantity of data from the detectors and present the
extrapolated results to the physicist.
Accelerators and particle detectors - How do we experiment with tiny particles? - Detector shapes
Physicists are curious about the events that
occur during and after a particle's collision.
For this reason, they place detectors in the regions
which will be showered with particles following an event.
Detectors are built in different ways
according to the type of collision they analyize.
Fixed Target: With a fixed-target experiment the particles produced
generally fly in the forward direction, so detectors are cone shaped and are
placed "downstream."
Colliding Beams:
During a colliding-beam experiment, the particles radiate in
all directions, so the detector is spherical or, more commonly, cylindrical.
Accelerators and particle detectors - How do we interpret our data? - Modern detectors
Modern detectors consist of many
different pieces of equipment which test for
different aspects of an event. These many components are arranged in such a
way that physicists can obtain the most data about the
particles spawned by an
event.
This is a schematic design of a typical modern detector.
For more information:
Tracking chamber: The inner region of the detector is filled with highly segmented sensing
devices of various kinds, so that charged particle trajectories can be
very accurately determined.
Electromagnetic Calorimeter: This device measures the total energy of e+, e-, and photons. These
particles produce showers of e+/e- pairs in the material. The
e-'s (or e+'s) are deflected by the electric fields of atoms, causing
them to radiate photons. The photons then make e-/e+ pairs, which then
radiate photons, etc. The number of final e+, e- pairs is proportional
to the energy of the initiating particle.
Hadron Calorimeter: This device measures the total energy of hadrons. The hadrons interact
with the dense material in this region, producing a shower of charged
particles. The energy that these charged particles deposit is then
measured.
Muon Chambers: Only muons and neutrinos get this far. The muons are detected, but the
weakly interacting neutrinos escape. The presence of neutrinos can be
inferred by the "missing" energy.
Magnet: The path of a charged particle curves in a magnetic field. The radius
of curvature and direction tell the momentum and the sign of the charge.
The reason that detectors are divided into many components is that each
component tests for a special set of particle properties.
These components are
stacked so that all particles will go through the different
layers sequentially. A
particle will not be evident until
it either interacts with the detector in a
measurable fashion, or decays into detectable particles.
Accelerators and particle detectors - How do we interpret our data? - Typical detector components
The reason that detectors are divided into many components is that each
component tests for a special set of particle properties.
These components are
stacked so that all particles will go through the different
layers sequentially. A
particle will not be evident until
it either interacts with the detector in a
measurable fashion, or decays into detectable particles.
The interaction of various particles with the different
components of a detector:
*Neutrinos are not shown on this chart because they
rarely interact with matter,
and can only be detected by missing matter and energy.
Just so you know, the pion
() is a charged
meson.*
A few important things to note:
Charged particles, like
electrons and
protons,
are detected both in the tracking
chamber and the electromagnetic calorimeter.
Neutral particles, like
neutrons and
photons,
are not detectable in the tracking chamber;
they are only evident when they interact with the detector.
Photons are detected by the electromagnetic calorimeter,
while neutrons are evidenced by the energy they deposit in the
hadron calorimeter.
Each particle type has its own "signature" in the detector.
For example, if a physicist detects a particle
only in the electromagnetic calorimeter,
then he is fairly certain that he observed a photon.
An electron and a positron
were produced when a particle and its antiparticle
collided head-on, perpendicular to this screen.
What conservation law APPEARS to have been broken?
Charge?
Number of Leptons?
Momentum?
Energy?
Answer: The conservation of momentum appears to be violated, but there were
unseen neutrinos.
Accelerators and particle detectors - How do we interpret our data? - Measuring charge and momentum
One important function of the detector is to measure a particle's charge and
momentum.
For this reason, the inner parts of the detector,
especially the tracking device,
are in a strong magnetic field.
The signs of the charged particles can easily be
read from their paths, since
positive and negative particles curve in opposite
directions in the same magnetic field.
The momenta of particles can be calculated since the paths of particles with
greater momentum bend less than those of lesser momentum. This is
because a particle with greater momentum will spend less time in the
magnetic field or have
greater inertia than the particle
with lesser momentum, and thus bends less in a magnetic field.
Accelerators and particle detectors - How do we interpret our data? - Detector cross section
To give you an idea of the paths that particles
will take through a detector,
here is a cross-section view of a detector,
looking down the tube the colliding
beams come from. Note the different places where various particles will be
detected.
Physicists can figure out the type of particle based on where that particle appeared in the detector.
Accelerators and particle detectors - How do we interpret our data? - Quiz - Particle tracks
These next 6 event pictures are from a modern detector and show
some of the possible decays of a Z particle. (The Z decays in too
short a time to be seen.) Try to identify the particles that left these
tracks. If you need help, go back to the
detector components or
go back to the
detector end-view.
The Z particles also decay into certain particles
that then decay into the particles whose tracks
are seen here. What are the secondary
particles and the final ones?
3: One example:Z --> a tau+ and tau-. Then the tau+ --> a muon+ and
neutrinos, and the tau- --> an electron and neutrinos, or with
opposite signs.
The Standard Model - What holds it together? - Quark confinement
6: The same thing happens as in #5, except that a radiated gluon causes the third cluster of particles.
Accelerators and particle detectors - How do we interpret our data? - The computer reconstruction
Detectors record millions of points of data during collision events.
For this reason, it is necessary to let a computer look at this data,
and figure out the
most likely particle paths and decays, as well as anomalies from the
expected behavior.
This is a computer reconstruction of a proton-antiproton
collision event that
produced an electron-positron pair as well as many other particles. This
particular event, and many other like it, provided evidence
for the Z boson,
one of the carrier particles for the collision producing top quarks.
It is through analysis of events like these that physicists have found
evidence for the Standard Model.
Accelerators and particle detectors - How do we interpret our data? - A quark/gluon event
In these pictures
e- and e+
beams, perpendicular to the screen, met and
annihilated. The resulting quarks and antiquarks combined to produce
mesons and baryons (The Standard Model - What is the world made of? - Hadrons, Baryons, and Mesons ), whose tracks are shown.
On the left, three clusters, initiated by a quark,
its antiquark, and a gluon,
provide evidence for the existence of gluons. On the right, two back-to-back
clusters of particles were initiated by a quark and its oppositely moving
antiquark.
Accelerators and particle detectors - How do we interpret our data? - The end
Now you have seen the techniques used to examine the experimental
evidence which supports the Standard Model.
To summarize, physicists use accelerators to "peek" into the structure of particles.
Detectors collect data which is then analyzed by computers
and then by people.
)
+),
which is made of an up quark and
a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (
).
Photons
of different energies span the electromagnetic
spectrum of x rays, visible light, radio waves, and so forth.
1/3 and 
2/3 
(alpha),
(beta), and
leaves no track.
A Circle 
Fixed target: Shoot a particle at a fixed target.
Linacs: Linear accelerators,
in which the particle starts at one end and comes out the other.
Synchrotrons: Accelerators built in a circle,
in which the particle goes around and around and around...
) is a charged
meson.*
For this reason, the inner parts of the detector,
especially the tracking device,
are in a strong magnetic field.
The signs of the charged particles can easily be
read from their paths, since
positive and negative particles curve in opposite
directions in the same magnetic field.
The momenta of particles can be calculated since the paths of particles with
greater momentum bend less than those of lesser momentum. This is
because a particle with greater momentum will spend less time in the
magnetic field or have
greater inertia than the particle
with lesser momentum, and thus bends less in a magnetic field.
2: muon+, muon-
4: Similarly, Z --> a tau+ and a tau-. Then the tau+ --> a muon+
and neutrinos, and the tau- --> hadrons and neutrinos, or with
opposite signs.
5: The Z particle becomes a quark and its antiquark. The energy in the
strong-force field produces more quark-antiquark pairs, which then form
hadrons.
