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.
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-).
Atom (Wikipedia)
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. 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 Greek ἄτομος (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).
atomic structure of matter (2)
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Periodic Table (Wikipedia)
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.
So, then, are protons and neutrons fundamental?
Atomic Nucleus (Wikipedia)
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.
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.
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 subatomic hadron 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.)
mmmmmmmmmmmm
ATOM MODEL
Rutherford model (Wikipedia)
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).
Bohr atom model (Wikipedia)
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).Rutherford-Bohr-Sommerfield atom model
Vector model of the atom (Wikipedia)
The most important quantity is the total atomic angular momentum J, not including the nuclear spin I. [1]
In physics, in particular quantum mechanics, the Vector model of the atom is a model of the atom in terms of angular momentum [2]. It can be considered as the extension of Rutherford-Bohr-Sommerfield atom model to multi-electron atoms.
Illustration of the vector model of orbital angular momentum.This is the modern atom model.
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
Subido por Best0fScience el 19/11/2009. http://www.facebook.com/ScienceReason ... The standard model of Particle Physics (Chapter 5): Electrons, Protons And Neutrons.
Scale of the atom.
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?)
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.
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!"
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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.
Fermions
The Standard Model includes 12 elementary particles of spin
known as fermions. According to the spin-statistics theorem, fermions respect the Pauli exclusion principle. Each fermion has a corresponding antiparticle.The fermions of the Standard Model are classified according to how they interact (or equivalently, by what charges they carry). There are six quarks (up, down, charm, strange, top, bottom), and six leptons (electron, electron neutrino, muon, muon neutrino, tau, tau neutrino). Pairs from each classification are grouped together to form a generation, with corresponding particles exhibiting similar physical behavior (see table).
| Charge | First generation | Second generation | Third generation | ||||
|---|---|---|---|---|---|---|---|
| Quarks | +2/3 | Up | u | Charm | c | Top | t |
| −1/3 | Down | d | Strange | s | Bottom | b | |
| Leptons | −1 | Electron | e− | Muon | μ− | Tau | τ− |
| 0 | Electron neutrino | ν e |
Muon neutrino | ν μ |
Tau neutrino | ν τ |
|
CERN: The Standard Model Of Particle Physics
Subido por Best0fScience el 22/07/2010 http://www.facebook.com/ScienceReason ... The Standard Model Of Particle Physics. This film was produced as part of the CERN/ATLAS multimedia contest internship.
mmmmmmmmmmmmmmmmmmmmmmmmmmmmm
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.
| 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! |
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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.
What is Antimatter?
Subido por
fermilab
el 14/11/2011
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?
Subido por
SLAC
el 26/01/2011
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.
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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. |
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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? |
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There are six flavors of quarks. "Flavors" just means different kinds. The two lightest are called up and downhe | |||||
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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 fourth quark type, the charm quark, was named on a whim. It was discovered in 1974 almost simultaneously at both the Stanford Linear Accelerator Center (SLAC) and at Brookhaven National Laboratory. |
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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
and nuclei decay by beta decay in processes like 
 |
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.
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| *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. |
Murray Gell-Mann sobre la belleza y la verdad en la física
http://www.ted.com/talks/murray_gell_mann_on_beauty_and_truth_in_physics.html
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?
Translated into Spanish by Raul Saavedra Reviewed by Sebastian Betti
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).
Quarks have various intrinsic properties, including electric charge, color charge, spin, and mass. Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge. For every quark flavor there is a corresponding type of antiparticle, known as antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign.
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]
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| Like social elephants, quarks only exist in groups with other quarks and are never found alone.
Composite particles made of quarks are called There are two classes of hadrons (try putting your mouse on the elephants): Baryons and Mesons    ).
  +),
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:  |
Hadron (Wikipedia)
In particle physics, a hadron (Greek: ἁδρός, hadrós, "stout, thick") is a composite particle made of quarks held together by the strong force (as atoms and molecules are held together by the electromagnetic force). Hadrons are categorized into two families: baryons (made of three quarks) and mesons (made of one quark and one antiquark).The best-known hadrons are protons and neutrons (both baryons), which are components of atomic nuclei. All hadrons except protons are unstable and undergo particle decay–however neutrons are stable inside atomic nuclei. The best-known mesons are the pion and the kaon, which were discovered during cosmic ray experiments in the late 1940s and early 1950s. However, these are not the only hadrons; a great number of them have been discovered and continue to be discovered (see list of baryons and list of mesons).
Other types of hadron may exist, such as tetraquarks (or, more generally, exotic mesons) and pentaquarks (exotic baryons), but no current evidence conclusively suggests their existence. [1][2]
Baryon (Wikipedia)
A baryon is a composite particle made up of three quarks (as distinct from mesons, which comprise one quark and one antiquark). Baryons and mesons belong to the hadron family, 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]
Meson (Wikipedia)
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.
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| 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.  Trivia: "Lepton" comes from the Greek for "small mass," but this is a misnomer. Why? [Answer] |
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.
 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! |
| 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.
 |
Which lepton decays are possible? Why or why not?
|
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 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. |
Questions:What are electrons made of? Nothing! Electrons are fundamental, as far as we know. 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) Neutrinos
Subido por
sciencemadefun
el 10/04/2007
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. |
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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.
|
| 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.) 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.  |
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.
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!
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|
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
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. Quantum chromodynamics- QCD (Wikipedia) In theoretical physics, quantum chromodynamics (QCD) is a theory of the strong interaction (color force), a fundamental force describing the interactions of the quarks and gluons making up hadrons (such as the proton, neutron or pion). It is the study of the SU(3) Yang–Mills theory of color-charged fermions (the quarks). QCD is a quantum field theory of a special kind called a non-abelian gauge theory. It is an important part of the Standard Model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years. QCD enjoys two peculiar properties:
Unsolved problems in physics  QCD in the non-perturbative regime:
|
-
-
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.
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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.
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]
| Generation 1 | Generation 2 | Generation 3 | ||||||
|---|---|---|---|---|---|---|---|---|
| Fermion | Symbol | Weak isospin |
Fermion | Symbol | Weak isospin |
Fermion | Symbol | Weak isospin |
| Electron |  |
 |
Muon |  |
|
Tau |  |
|
| Electron neutrino |  |
 |
Muon neutrino |  |
|
Tau neutrino |  |
|
| Up quark |  |
|
Charm quark |  |
|
Top quark |  |
|
| Down quark |  |
|
Strange quark |  |
|
Bottom quark |  |
|
| All left-handed antiparticles have weak isospin of 0. Right-handed antiparticles have the opposite weak isospin. | ||||||||
| . | ||||||||
W and Z bosons (Wikipedia)
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 physicist Steven 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.
π+ decay through the weak interaction
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The Standard Model - What holds it together? -
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 1015 K, 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 Z gauge 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.
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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.
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.
(I still don't get it.)
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 force proportional 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.
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 spacetime curvature 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.
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional 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.
In general relativity, the effects of gravitation are ascribed to spacetime curvature 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.
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Fundamental Forces
The Electromagnetic 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.
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)
The Gravity Force
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.
lll -Exchange Particles- lll
Physics beyond the Standard Model
Beyond the Standard Model: CMS Higgs event
Illustration courtesy Fermilab,
D0 Experiment.
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The Standard Model - What holds it together? -
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.)
Standard Model
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Find out more about particle physics and the Standard Model!
The Electromagnetic Force
One of the four fundamental forces, the electromagnetic force manifests itself through the forces between charges (Coulomb's Law) and the magnetic force, both of which are summarized in the Lorentz force law. Fundamentally, both magnetic and electric forces are manifestations of an exchange force involving the exchange of photons . The quantum approach to the electromagnetic force is called quantum electrodynamics or QED. The electromagnetic force is a force of infinite range which obeys the inverse square law, and is of the same form as the gravity force.
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
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 Weak Force
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)
The Gravity Force
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). 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
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
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.
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
| 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. |
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.
 |
|
 |
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. |
|
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 charged vertices in the weak interaction with quarks take the form
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 neutral interaction is simpler to conceive, but rarely observed because it competes with the much stronger electromagnetic interaction and is masked by it. |
  |
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.
|
Graviton and the Gravity Force
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.
refers to the theoretical developments needed to explain the deficiencies of the Standard Model, such as the origin of mass, the strong CP problem, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy.[1] Another problem lies within the mathematical framework of the Standard Model itself – the Standard Model is inconsistent with that of general relativity to the point that one or both theories break down in their descriptions under certain conditions (for example within known space-time singularities like the Big Bang and black hole event horizons).
Theories that lie beyond the Standard Model include various extensions of the standard model through supersymmetry, such as the Minimal Supersymmetric Standard Model (MSSM) and Next-to-Minimal Supersymmetric Standard Model (NMSSM), or entirely novel explanations, such as string theory, M-theory and extra dimensions.
As these theories tend to reproduce the entirety of current phenomena,
the question of which theory is the right one, or at least the "best
step" towards a Theory of Everything, can only be settled via experiments and is one of the most active areas of research in both theoretical and experimental physic
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
Feynman diagrams showing two ways the Higgs boson might be produced at the LHC. Left: two gluons convert to top/anti-top quark pairs, which combine. Right: two quarks emit W or Z bosons, which combine.
The Higgs Boson
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.
Summary of interactions between particles described by the Standard Model.
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.
| 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.
-
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).
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:
= 1.055 x 10-34 J s. This is
Planck's Constant.
The Standard Model - What holds it together?
-
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.
1/3 and 2/3 for quarks).
Quantum Mechanics describes particle interactions.
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.
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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!)
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!)
The Pauli exclusion principle (Wikipedia) is the quantum mechanical principle that no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is anti-symmetric with respect to exchange of the particles. The principle was formulated by Austrian physicist Wolfgang Pauli in 1925.
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.
-
 BOSONS  
* The predicted graviton has a spin of 2.
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 relativistic quantum 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 relativistic quantum 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:
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of the Indian physicist Satyendra Nath Bose.[1] Bosons contrast with fermions, which obey Fermi–Dirac statistics. Two or more fermions cannot occupy the same quantum state. Since bosons with the same energy can occupy the same place in space, bosons are often force carrier particles. In contrast, fermions are usually associated with matter (although in quantum physics the distinction between the two concepts is not clear cut). Bosons may be either elementary, like photons, or composite, like mesons. Some composite bosons do not satisfy the criteria for Bose-Einstein statistics and are not truly bosons (e.g. helium-4 atoms); a more accurate term for such composite particles would be "bosonic-composites". All observed bosons have integer spin, as opposed to fermions, which have half-integer spin. This is in accordance with the spin-statistics theorem which states that in any reasonable relativistic quantum field theory, particles with integer spin are bosons, while particles with half-integer spin are fermions. While most bosons are composite particles, in the Standard Model, there are six bosons which are elementary:
Composite bosons are important in superfluidity and other applications of Bose–Einstein condensates. All observed elementary particles are either fermions or bosons. The observed elementary bosons are all gauge bosons: photons, W and Z bosons and gluons.
Finally, many approaches to quantum gravity postulate a force carrier for gravity, the graviton, which is a boson of spin 2. -
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 Standard Model - Particle decays and annihiliations
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 DecayParticle DecayIn 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 - 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".   (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
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?   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.  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.   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 -   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. 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:  , 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:  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.
 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 -
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. We have talked a lot about decays and annihilations, so let's now look at some examples of these processes.  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!) 
  
The Standard Model - Particle decays and annihiliations -  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. 
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   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? 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? 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.  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. 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.
 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.  No supersymmetric particle has yet been found, but experiments are underway at CERN and Fermilab to detect supersymmetric partner particles.   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?" 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!  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 fromgravitational 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.   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... 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.  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.  
 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.  
Look at answers
The First Target Was:
A Wedge The Second Target Was:
A Circle 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.  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. 
Accelerators and particle detectors - How do we detect what's happening? -
  
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:   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!
The bear's outline is fairly clear, so this is a "sharp" image. 
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.  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. 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?  High-energy physicists apply this principle when they use particle accelerators to increase the momentum of a probing particle, thus decreasing its wavelength. 
 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.
 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, 
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.   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 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.  
The above is an animation of the following concept:
 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. 
 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? - 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.
 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. ] 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. ]  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.  How can a physicist determine what happened if she can never record the presence of several key particles? 
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.  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.
This is a schematic design of a typical modern detector.  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. 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:
 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?  Number of Leptons? Momentum? Energy? Answer: The conservation of momentum appears to be violated, but there were unseen neutrinos. 
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.
 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.  1: e-, e+  2: muon+, muon-
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. 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.
How can a strong field produce quarks?
The Standard Model - What holds it together? -  6: The same thing happens as in #5, except that a radiated gluon causes the third cluster of particles.
It is through analysis of events like these that physicists have found evidence for 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.  |
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...
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