domingo, 2 de enero de 2011

The Standard Model: Particles and Forces




People have long asked:
"What is the world made of?" and "What holds it together?"
What is the World Made of?
Why do so many things in this world share the same characteristics?
People have come to realize that the matter of the world is made from a few fundamental building blocks of nature.
The word "fundamental" is key here. By fundamental building blocks we mean objects that are simple and structureless -- not made of anything smaller.
Even in ancient times, people sought to organize the world around them into fundamental elements, such as earth, air, fire, and water.














Today we know that there is something more fundamental than earth, water, air, and fire...

THE ATOM

By convention there is color,
By convention sweetness,
By convention bitterness,
But in reality there are atoms and space.

   -Democritus (c. 400 BCE)
Around 1900, people thought of atoms as permeable balls with bits of electric charge bouncing around inside.

 Atomic theory (Wikipedia)
In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity. It began as a philosophical concept in ancient Greece (Democritus) and India and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of particles.
The word "atom" (from the ancient Greek adjective atomos, 'indivisible'[1]) was applied to the basic particle that constituted a chemical element, because the chemists of the era believed that these were the fundamental particles of matter. However, around the turn of the 20th century, through various experiments with electromagnetism and radioactivity, physicists discovered that the so-called "indivisible atom" was actually a conglomerate of various subatomic particles (chiefly, electrons, protons and neutrons) which can exist separately from each other. In fact, in certain extreme environments such as neutron stars, extreme temperature and pressure prevents atoms from existing at all. Since atoms were found to be actually divisible, physicists later invented the term "elementary particles" to describe indivisible particles. The field of science which studies subatomic particles is particle physics, and it is in this field that physicists hope to discover the true fundamental nature of matter.


But is the atom fundamental?


 The Standard Model - What is fundamental? 

Is the Atom Fundamental?


People soon realized that they could categorize atoms into groups that shared similar chemical properties (as in the Periodic Table of the Elements). This indicated that atoms were made up of simpler building blocks, and that it was these simpler building blocks in different combinations that determined which atoms had which chemical properties.

Moreover, experiments which "looked" into an atom using particle probes indicated that atoms had structure and were not just squishy balls. These experiments helped scientists determine that atoms have a tiny but dense, positive nucleus and a cloud of negative electrons (e-).



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) 

Subido por el 19/01/2011. educational

 

 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.


Physicists have discovered that protons and neutrons are composed of even smaller particles called quarks.
As far as we know, quarks are like points in geometry. They're not made up of anything else.
After extensively testing this theory, scientists now suspect that quarks and the electron (and a few other things we'll see in a minute) are fundamental.

Protons (Wikipedia)
The proton is a subatomic particle with the symbol p or p+ and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.
In the standard model of particle physics, the proton is a hadron, composed of quarks. Prior to that model becoming a consensus in the physics community, the proton was considered a fundamental particle. A proton is composed of two up quarks and one down quark, and is about 1.6–1.7 fm in diameter.[2]
The free proton is stable and is found naturally in a number of situations. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate in vacuum for interstellar distances. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay, and also result from the decay of free neutrons, which are unstable. In all such cases, protons must lose sufficient velocity and (kinetic energy) to allow them to become associated with electrons, since this is a relatively low-energy interaction. However, in such an association, the character of the bound proton is not changed, and it remains a proton.
The attraction of low-energy protons to electrons, either free electrons or electrons as present in normal matter, causes such protons to soon form chemical bonds with atoms. This happens at sufficiently "cold" temperatures (comparable to temperatures at the surface of the Sun). In interaction with normal (non plasma) matter, low-velocity free protons are attracted to electrons in any atom or molecule with which they come in contact, causing them to combine. In vacuum, a sufficiently slow proton may pick up a free electron, becoming a neutral hydrogen atom, which then will then react chemically with other atoms if they are available and sufficiently cold.
 

 The quark structure of the proton. (The color assignment of individual quarks is not important, only that all three colors are present.)

 

 

 Neutrons (Wikipedia)

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 ().[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.

Electrons are in constant motion around the nucleus, protons and neutrons jiggle within the nucleus, and quarks jiggle within the protons and neutrons.
This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space!


Electrons, Protons And Neutrons | Standard Model Of Particle Physics

 Subido por 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?)
What are we looking for?

Physicists constantly look for new particles. When they find them, they categorize them and try to find patterns that tell us about how the fundamental building blocks of the universe interact.
We have now discovered about two hundred particles (most of which aren't fundamental). To keep track of all of these particles, they are named with letters from the Greek and Roman alphabets.
Of course, the names of particles are but a small part of any physical theory. You should not be discouraged if you have trouble remembering them. Take heart: even the great Enrico Fermi once said to his student (and future Nobel Laureate) Leon Lederman, "Young man, if I could remember the names of these particles, I would have been a botanist!"


 The standard model














Physicists have developed a theory called The Standard Model that explains what the world is and what holds it together. It is a simple and comprehensive theory that explains all the hundreds of particles and complex interactions with only:
  • 6 quarks.
  • 6 leptons. The best-known lepton is the electron. We will talk about leptons in just a few pages.
  • Force carrier particles, like the photon. We will talk about these particles later.
All the known matter particles are composites of quarks and leptons, and they interact by exchanging force carrier particles.





The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon experimental confirmation of the existence of quarks. Since then, discoveries of the bottom quark (1977), the top quark (1995) and the tau neutrino (2000) have given further credence to the Standard Model. Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a theory of almost everything.
Still, the Standard Model falls short of being a complete theory of fundamental interactions because it does not incorporate the physics of dark energy nor of the full theory of gravitation as described by general relativity. The theory does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not correctly account for neutrino oscillations (and their non-zero masses). Although the Standard Model is theoretically self-consistent, it has several apparently unnatural properties giving rise to puzzles like the strong CP problem and the hierarchy problem.
Nevertheless, the Standard Model is important to theoretical and experimental particle physicists alike. For theorists, the Standard Model is a paradigmatic example of a quantum field theory, which exhibits a wide range of physics including spontaneous symmetry breaking, anomalies, non-perturbative behavior, etc. It is used as a basis for building more exotic models which incorporate hypothetical particles, extra dimensions and elaborate symmetries (such as supersymmetry) in an attempt to explain experimental results at variance with the Standard Model, such as the existence of dark matter and neutrino oscillations. In turn, experimenters have incorporated the standard model into simulators to help search for new physics beyond the Standard Model.
Recently, the standard model has found applications in fields besides particle physics, such as astrophysics, cosmology, and nuclear physics.
 The Standard Model of elementary particles, with the gauge bosons in the rightmost column

Fermions
The Standard Model includes 12 elementary particles of spin {}_\frac{1}{2} 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).

Organization of Fermions

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












Standard model of particle physics (Feynmann Diagram - Gluon Radiation)


  Summary of the interactions between particles included in the Standard Model

CERN: The Standard Model Of Particle Physics 

 Subido por 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.







Matter and antimatter

For every type of matter particle we've found, there also exists a corresponding antimatter particle, or antiparticle. Antiparticles look and behave just like their corresponding matter particles, except they have opposite charges. For instance, a proton is electrically positive whereas an antiproton is electrically negative. Gravity affects matter and antimatter the same way because gravity is not a charged property and a matter particle has the same mass as its antiparticle.
When a matter particle and antimatter particle meet, they annihilate into pure energy!






bubble chamber photo





Bubble chambers were an important kind of particle detector from 1953 well into the 1970s. The idea behind a bubble chamber is that when you shoot charged particles into a superheated liquid, the particles will leave behind a track of bubbles. This makes it easy to track the particles and figure out important things like their charge and mass. (A superheated liquid is made by lowering the pressure in the chamber when the liquid is just below the boiling point.)
The magnetic field in this chamber
In order to find out things about particles, physicists measure their charge and momentum. To do this they observe particle collisions in strong magnetic fields, because different kinds of particles behave very differently in a magnetic field depending on their charge and their momentum.
 
For one thing, the signs of charged particles can easily be read from their paths, since they curve in opposite directions in the same magnetic field.
For another, the momenta of particles can be calculated easily because the path of a particle with greater momentum bends less than that of one with lesser momentum.
















Antimatter (Wikipedia)
In particle physics, antimatter is the extension of the concept of the antiparticle to matter, where antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example, a positron (the antiparticle of the electron or e+) and an antiproton (p) can form an antihydrogen atom in the same way that an electron and a proton form a "normal matter" hydrogen atom. Furthermore, mixing matter and antimatter can lead to the annihilation of both, in the same way that mixing antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle–antiparticle pairs. The result of antimatter meeting matter is an explosion.[1]
There is considerable speculation as to why the observable universe is apparently composed almost entirely of matter (as opposed to a mixture of matter and antimatter), whether there exist other places that are almost entirely composed of antimatter instead, and what sorts of technology might be possible if antimatter could be harnessed. At this time, the apparent asymmetry of matter and antimatter in the visible universe is one of the greatest unsolved problems in physics. The process by which this asymmetry between particles and antiparticles developed is called baryogenesis.

 What is Antimatter?

Subido por 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 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.


mmmmmmmmmmmmmmmm 


Quarks



Quarks
are one type of matter particle. Most of the matter we see around us is made from protons and neutrons, which are composed of quarks.
There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.) Be glad that quarks have such silly names -- it makes them easier to remember!


Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively. Quarks also carry another type of charge called color charge, which we will discuss later.

The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years.
Want to see a particle physicist's idea of a good pun?




Charm
Up Top
Down Bottom

Strange

 Cork model/bad pun by Don Groom, Particle Data Group, LBNL.

 

The naming of quarks...

...began when, in 1964, Murray Gell-Mann and George Zweig suggested that hundreds of the particles known at the time could be explained as combinations of just three fundamental particles. Gell-Mann chose the name "quarks," pronounced "kworks," for these three particles, a nonsense word used by James Joyce in the novel Finnegan's Wake:

"Three quarks for Muster Mark!"

In order to make their calculations work, the quarks had to be assigned fractional electrical charges of 2/3 and -1/3. Such charges had never been observed before. Quarks are never observed by themselves, and so initially these quarks were regarded as mathematical fiction. Experiments have since convinced physicists that not only do quarks exist, but there are six of them, not three.



How did quarks get their silly names?






There are six flavors of quarks. "Flavors" just means different kinds.
The two lightest are called up and downhe




The third quark is called strange. It  was named after the "strangely" long lifetime of the K particle, the first composite particle found to contain this quark.

The 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.



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













is thought to be the result of a more fundamental quark process














 

The Strange Quark: In 1947 during a study of cosmic ray interactions, a product of a proton collision with a nucleus was found to live for much longer time than expected: 10-10 seconds instead of the expected 10-23 seconds! This particle was named the lambda particle (Λ0) and the property which caused it to live so long was dubbed "strangeness" and that name stuck to be the name of one of the quarks from which the lambda particle is constructed. The lambda is a baryon which is made up of three quarks: an up, a down and a strange quark.

The omega-minus, a baryon composed of three strange quarks, is a classic example of the need for the property called "color" in describing particles. Since quarks are fermions with spin 1/2, they must obey the Pauli exclusion principle and cannot exist in identical states. So with three strange quarks, the property which distinguishes them must be capable of at least three distinct values. 

The Charm Quark: In 1974 a meson called the J/Psi particle was discovered. With a mass of 3100 MeV, over three times that of the proton, this particle was the first example of another quark, called the charm quark. The J/Psi is made up of a charm-anticharm quark pair.The lightest meson which contains a charm quark is the D meson. It provides interesting examples of decay since the charm quark must be transformed into a strange quark by the weak interaction in order for it to decay.One baryon with a charm quark is a called a lambda with symbol Λ+c . It has a composition udc and a mass of 2281 MeV/c2.

The Top Quark: Convincing evidence for the observation of the top quark was reported by Fermilab 's Tevatron facility in April 1995. The evidence was found in the collision products of 0.9 TeV protons with equally energetic antiprotons in the proton-antiproton collider. The evidence involved analysis of trillions of 1.8 TeV proton-antiproton collisions. The Collider Detector Facility group had found 56 top candidates over a predicted background of 23 and the D0 group found 17 events over a predicted background of 3.8. The value for the top quark mass from the combined data of the two groups after the completion of the run was 174.3 +/- 5.1 GeV. This is over 180 times the mass of a proton and about twice the mass of the next heaviest fundamental particle, the Z0 vector boson at about 93 GeV.

The Bottom Quark: In 1977, an experimental group at Fermilab led by Leon Lederman discovered a new resonance at 9.4 GeV/c^2 which was interpreted as a bottom-antibottom quark pair and called the Upsilon meson.

 Table of quark properties
Quarks and Leptons are the building blocks which build up matter, i.e., they are seen as the "elementary particles". In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (over 200). The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. Quarks are observed to occur only in combinations of two quarks (mesons), three quarks (baryons). There was a recent claim of observation of particles with five quarks (pentaquark), but further experimentation has not borne it out.




quarkSymbolSpinChargeBaryon
Number
SCBTMass*














Up














U














1/2














+2/3














1/3














0














0














0














0














1.7-3.3 MeV














Down














D














1/2














-1/3














1/3














0














0














0














0














4.1-5.8 MeV














Charm














C














1/2














+2/3














1/3














0














+1














0














0














1270 MeV














Strange














S














1/2














-1/3














1/3














-1














0














0














0














101 MeV














Top














T














1/2














+2/3














1/3














0














0














0














+1














172 GeV














Bottom














B














1/2














-1/3














1/3














0














0














-1














0














4.19 GeV(MS)
4.67 GeV(1S)

*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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Hadrons, Baryons, and Mesons











Like social elephants, quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called















HADRONS
Although individual quarks have fractional electrical charges, they combine such that hadrons have a net integer electric charge. Another property of hadrons is that they have no net color charge even though the quarks themselves carry color charge (we will talk more about this later).
There are two classes of hadrons (try putting your mouse on the elephants): Baryons and Mesons














BARYONS
...are any hadron which is made of three quarks (qqq).
Because they are made of two up quarks and one down quark (uud), protons are baryons.
So are neutrons (udd).













 
MESONS
...contain one quark (q) and one antiquark ().
One example of a meson is a pion (+), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (-) is made up a down quark and an up antiquark.
Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K-) meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components.

A weird thing about hadrons
is that only a very very very small part of the mass of a hadron is due to the quarks in it. For example, a proton (uud) has more mass than the sum of the masses of its quarks:














Most of the mass we observe in a hadron comes from its kinetic and potential energy. These energies are converted into the mass of the hadron as described by Einstein's equation that relates energy and mass, E = mc2.

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 23 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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Leptons
The other type of matter particles are the leptons.
There are six leptons, three of which have electrical charge and three of which do not. They appear to be point-like particles without internal structure. The best known lepton is the electron (e-). The other two charged leptons are the muon() and the tau(), which are charged like electrons but have a lot more mass. The other leptons are the three types of neutrinos (). They have no electrical charge, very little mass, and they are very hard to find.
Quarks are sociable and only exist in composite particles with other quarks, whereas leptons are solitary particles. Think of the charged leptons as independent cats with associated neutrino fleas, which are very hard to see.














For each lepton there is a corresponding antimatter antilepton. Note that the anti-electron has a special name, the "positron."

Trivia: "Lepton" comes from the Greek for "small mass," but this is a misnomer. Why? [Answer]
Answer: Even though "lepton" comes from the Greek for "small mass", the tau lepton is more than 3000 times as massive as the electron.


 Lepton decays
The heavier leptons, the muon and the tau, are not found in ordinary matter at all. This is because when they are produced they very quickly decay, or transform, into lighter leptons. Sometimes the tau lepton will decay into a quark, an antiquark, and a tau neutrino. Electrons and the three kinds of neutrinos are stable and thus the types we commonly see around us.














When a heavy lepton decays, one of the particles it decays into is always its corresponding neutrino. The other particles could be a quark and its antiquark, or another lepton and its antineutrino.
Physicists have observed that some types of lepton decays are possible and some are not. In order to explain this, they divided the leptons into three lepton families: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino. The number of members in each family must remain constant in a decay. (A particle and an antiparticle in the same family "cancel out" to make the total of them equal zero.)
Although leptons are solitary, they are always loyal to their families!

 Lepton type conservation
Leptons are divided into three lepton families: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino. We use the terms "electron number," "muon number," and "tau number" to refer to the lepton family of a particle. Electrons and their neutrinos have electron number +1, positrons and their antineutrinos have electron number -1, and all other particles have electron number 0. Muon number and tau number operate analogously with the other two lepton families.
One important thing about leptons, then, is that electron number, muon number, and tau number are always conserved when a massive lepton decays into smaller ones.

Let's take an example decay.

A muon decays into a muon neutrino, an electron, and an electron antineutrino:
As you can see, electron, muon, and tau numbers are conserved. These and other conservation laws are what we believe define whether or not a given hypothetical lepton decay is possible. 

Lepton decay quiz

Which lepton decays are possible? Why or why not?


(A tau lepton decays into an electron, an electron antineutrino, and a tau neutrino.)
Yes! Charge, tau number, electron number, and energy are all conserved.
 
 
(A tau lepton decays into a muon and a tau neutrino.)
 No! Muon number is not conserved. A muon has a muon number of 1, and thus the right side of the decay equation has muon number 1, but the left side has muon number of 0.

Now try a tricky one:

 
(An electron decays into a muon, a muon antineutrino, and an electron neutrino.)
No! Surprise! Although electron and muon numbers are both conserved, energy is not conserved. A muon has a lot more mass than an electron, and a lepton cannot decay into something more massive than it started out!

 Leptons (Wikipedia)

A lepton is an elementary particle and a fundamental constituent of matter.[1] The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms and positronium, while neutrinos rarely interact with anything, and are consequently rarely observed.
There are six types of leptons, known as flavours, forming three generations.[2] The first generation is the electronic leptons, comprising the electron (e) and electron neutrino (ν
e
); the second is the muonic leptons, comprising the muon (μ) and muon neutrino (ν
μ
); and the third is the tauonic leptons, comprising the tau (τ) and the tau neutrino (ν
τ
). Electrons have the least mass of all the charged leptons. The heavier muons and taus will rapidly change into electrons through a process of particle decay: the transformation from a higher mass state to a lower mass state. Thus electrons are stable and the most common charged lepton in the universe, whereas muons and taus can only be produced in high energy collisions (such as those involving cosmic rays and those carried out in particle accelerators).
Leptons have various intrinsic properties, including electric charge, spin, and mass. Unlike quarks however, leptons are not subject to the strong interaction, but they are subject to the other three fundamental interactions: gravitation, electromagnetism (excluding neutrinos, which are electrically neutral), and the weak interaction. For every lepton flavor there is a corresponding type of antiparticle, known as antilepton, that differs from the lepton only in that some of its properties have equal magnitude but opposite sign. However, according to certain theories, neutrinos may be their own antiparticle, but it is not currently known whether this is the case or not.
The first charged lepton, the electron, was theorized in the mid-19th century by several scientists[3][4][5] and was discovered in 1897 by J. J. Thomson.[6] The next lepton to be observed was the muon, discovered by Carl D. Anderson in 1936, but it was erroneously classified as a meson at the time.[7] After investigation, it was realized that the muon did not have the expected properties of a meson, but rather behaved like an electron, only with higher mass. It took until 1947 for the concept of "leptons" as a family of particle to be proposed.[8] The first neutrino, the electron neutrino, was proposed by Wolfgang Pauli in 1930 to explain certain characteristics of beta decay.[8] It was first observed in the Cowan–Reines neutrino experiment conducted by Clyde Cowan and Frederick Reines in 1956.[8][9] The muon neutrino was discovered in 1962 by Leon M. Lederman, Melvin Schwartz and Jack Steinberger,[10] and the tau discovered between 1974 and 1977 by Martin Lewis Perl and his colleagues from the Stanford Linear Accelerator Center and Lawrence Berkeley National Laboratory.[11] The tau neutrino remained elusive until July 2000, when the DONUT collaboration from Fermilab announced its discovery.[12][13]
Leptons are an important part of the Standard Model. Electrons are one of the components of atoms, alongside protons and neutrons. Exotic atoms with muons and taus instead of electrons can also be synthesized, as well as lepton–antilepton particles such as positronium.


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Neutrinos
Neutrinos are, as we've said, a type of lepton. Since they have no electrical or strong charge they almost never interact with any other particles. Most neutrinos pass right through the earth without ever interacting with a single atom of it. Neutrinos are produced in a variety of interactions, especially in particle decays. In fact, it was through a careful study of radioactive decays that physicists hypothesized the neutrino's existence.
 

For example: (1) In a radioactive nucleus, a neutron at rest (zero momentum) decays, releasing a proton and an electron. (2) Because of the law of conservation of momentum, the resulting products of the decay must have a total momentum of zero, which the observed proton and electron clearly do not. (3) Therefore, we need to infer the presence of another particle with appropriate momentum to balance the event. (4) We hypothesize that an antineutrino was released; experiments have confirmed that this is indeed what happens.
Because neutrinos were produced in great abundance in the early universe and rarely interact with matter, there are a lot of them in the Universe. Their tiny mass but huge numbers may contribute to total mass of the universe and affect its expansion.

 Quiz - What particles are made of



Questions:

What are protons made of? Protons are made of two up quarks and one down quark, expressed as uud.
 
What are electrons made of? Nothing! Electrons are fundamental, as far as we know.
 
Neutrinos (Wikipedia)
A neutrino  is an electrically neutral, weakly interacting elementary subatomic particle[1] with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected. The neutrino (meaning "small neutral one" in Italian) is denoted by the Greek letter ν (nu).
Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are affected only by the weak sub-atomic force, of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale, and are therefore able to travel great distances through matter without being affected by it.
Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Each type also has a corresponding antiparticle, called an antineutrino with an opposite chirality.
Most neutrinos passing through the Earth emanate from the Sun. About 65 billion (6.5×1010) solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.[2]
In September 2011, neutrinos apparently moving faster than light were detected (see OPERA neutrino anomaly). Since then the experiment has undergone extensive critique and efforts to replicate the results because confirming the results would change our understanding of the theory of relativity. In November 2011, the experiment was refined and yielded the same result. (See Speed below)

Neutrinos

 

Subido por 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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Now we think we have a good idea of what the world is made of: quarks and leptons. So...

What holds it together?

The universe, which we know and love, exists because the fundamental particles interact. These interactions include attractive and repulsive forces, decay, and annihilation.
There are four fundamental interactions between particles, and all forces in the world can be attributed to these four interactions!
That's right: Any force you can think of -- friction, magnetism, gravity, nuclear decay, and so on -- is caused by one of these four fundamental interactions.
What's the difference between a force and an interaction?
This is a hard distinction to make. Strictly speaking, a force is the effect on a particle due to the presence of other particles. The interactions of a particle include all the forces that affect it, but also include decays and annihilations that the particle might go through. (We will spend the next chapter discussing these decays and annihilations in more depth.)
The reason this gets confusing is that most people, even most physicists, usually use "force" and "interaction" interchangeably, although "interaction" is more correct. For instance, we call the particles which carry the interactions force carrier particles. You will usually be okay using the terms interchangeably, but you should know that they are different.





One tricky question that plagued physicists for many years was...

How do matter particles interact?

The problem is that things interact without touching! How do two magnets "feel" each other's presence and attract or repel accordingly? How does the sun attract the earth?
We know the answers to these questions are "magnetism" and "gravity," but what are these forces?
At a fundamental level, a force isn't just something that happens to particles. It is a thing which is passed between two particles.





The unseen effect
You can think about forces as being analogous to the following situation: (Flash)
Two people are standing on an ice pond. One person moves their arm and is pushed backwards; a moment later the other person grabs at an invisible object and is driven backwards. Even though you cannot see a basketball, you can assume that one person threw a basketball to the other person because you see its effect on the people. (Click on the checkmark or cross below the animation in order to make the basketball appear or disappear.)













It turns out that all interactions which affect matter particles are due to an exchange of force carrier particles, a different type of particle altogether. These particles are like basketballs tossed between matter particles (which are like the basketball players). What we normally think of as "forces" are actually the effects of force carrier particles on matter particles.
The basketball animation is, of course, a very crude analogy since it can only explain repulsive forces and gives no hint of how exchanging particles can result in attractive forces.
We see examples of attractive forces in everyday life (such as magnets and gravity), and so we generally take it for granted that an object's presence can just affect another object. It is when we approach the deeper question, "How can two objects affect one another without touching?" that we propose that the invisible force could be an exchange of force carrier particles. Particle physicists have found that we can explain the force of one particle acting on another to INCREDIBLE precision by the exchange of these force carrier particles.
One important thing to know about force carriers is that a particular force carrier particle can only be absorbed or produced by a matter particle which is affected by that particular force. For instance, electrons and protons have electric charge, so they can produce and absorb the electromagnetic force carrier, the photon. Neutrinos, on the other hand, have no electric charge, so they cannot absorb or produce photons.





























ELECTROMAGNETISM (E-M force)
 
The electromagnetic force causes like-charged things to repel and oppositely-charged things to attract. Many everyday forces, such as friction, and even magnetism, are caused by the electromagnetic, or E-M force. For instance, the force that keeps you from falling through the floor is the electromagnetic force which causes the atoms making up the matter in your feet and the floor to resist being displaced.



The carrier particle of the electromagnetic force is the photon ().  Photons of different energies span the electromagnetic spectrum of x rays, visible light, radio waves, and so forth.
Photons have zero mass, as far as we know, and always travel at the "speed of light", c, which is about 300,000,000 meters per second, or 186,000 miles per second, in a vacuum.

 Residual EM force
Atoms usually have the same numbers of protons and electrons. They are electrically neutral, therefore, because the positive protons cancel out the negative electrons. Since they are neutral, what causes them to stick together to form stable molecules? The answer is a bit strange: we've discovered that the charged parts of one atom can interact with the charged parts of another atom. This allows different atoms to bind together, an effect called the residual electromagnetic force.
 

So the electromagnetic force is what allows atoms to bond and form molecules, allowing the world to stay together and create the matter you interact with all of the time. Amazing, isn't it? All the structures of the world exist simply because protons and electrons have opposite charges!
See? Now you know the meaning of life!
 What about the nucleus?
We have another problem with atoms, though. What binds the nucleus together?  The nucleus of an atom consists of a bunch of protons and neutrons crammed together. Since neutrons have no charge and the positively-charged protons repel one another, why doesn't the nucleus blow apart?



 We cannot account for the nucleus staying together with just electromagnetic force. What else could there be? Gravity? Nope! The gravitational force is far too weak to overpower the electromagnetic force.
So how can we account for this dilemma?

 Strong

STRONG
To understand what is happening inside the nucleus, we need to understand more about the quarks that make up the protons and neutrons in the nucleus. Quarks have electromagnetic charge, and they also have an altogether different kind of charge called color charge. The force between color-charged particles is very strong, so this force is "creatively" called 


















STRONG
The strong force holds quarks together to form hadrons, so its carrier particles are whimsically called gluons because they so tightly "glue" quarks together. (Other name candidates included the "hold-on," the "duct-tape-it-on," and the "tie-it-on!")

Color charge behaves differently than electromagnetic charge. Gluons, themselves, have color charge, which is weird and not at all like photons which do not have electromagnetic charge. And while quarks have color charge, composite particles made out of quarks have no net color charge (they are color neutral). For this reason, the strong force only takes place on the really small level of quark interactions, which is why you are not aware of the strong force in your everyday life.

 Strong interaction (Wikipedia)
In particle physics, the strong interaction (also called the strong force, strong nuclear force, or color force) is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. At atomic scale, it is about 100 times stronger than electromagnetism, which in turn is orders of magnitude stronger than the weak force interaction and gravitation.
The strong interaction is observable in two areas: on a larger scale (about 1 to 3 femtometers (fm)), it is the force that binds protons and neutrons together to form the nucleus of an atom. On the smaller scale (less than about 0.8 fm, the radius of a nucleon), it is also the force (carried by gluons) that holds quarks together to form protons, neutrons and other hadron particles.
In the context of binding protons and neutrons (nucleons) together to form atoms, the strong interaction is called the nuclear force (or residual strong force). In this case, it is the residuum of the strong interaction between the quarks that make up the protons and neutrons. As such, the residual strong interaction obeys a quite different distance-dependent behavior between nucleons, from when it is acting to bind quarks within nucleons.
The strong interaction is thought to be mediated by gluons, acting upon quarks, antiquarks, and other gluons. Gluons, in turn, are thought to interact with quarks and gluons because all carry a type of charge called "color charge." Color charge is analogous to electromagnetic charge, but it comes in three types not two, and it results in a different type of force, with different rules of behavior. These rules are detailed in the theory of quantum chromodynamics (QCD), which is the theory of quark-gluon interactions.

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:
  • Confinement, which means that the force between quarks does not diminish as they are separated. Because of this, it would take an infinite amount of energy to separate two quarks; they are forever bound into hadrons such as the proton and the neutron. Although analytically unproven, confinement is widely believed to be true because it explains the consistent failure of free quark searches, and it is easy to demonstrate in lattice QCD.
  • Asymptotic freedom, which means that in very high-energy reactions, quarks and gluons interact very weakly. This prediction of QCD was first discovered in the early 1970s by David Politzer and by Frank Wilczek and David Gross. For this work they were awarded the 2004 Nobel Prize in Physics.
There is no known phase-transition line separating these two properties; confinement is dominant in low-energy scales but, as energy increases, asymptotic freedom becomes dominant.
Unsolved problems in physics
 QCD in the non-perturbative regime: 

 
Quarks and gluons are color-charged particles. Just as electrically-charged particles interact by exchanging photons in electromagnetic interactions, color-charged particles exchange gluons in strong interactions. When two quarks are close to one another, they exchange gluons and create a very strong color force field that binds the quarks together. The force field gets stronger as the quarks get further apart. Quarks constantly change their color charges as they exchange gluons with other quarks.

How does color charge work?

There are three color charges and three corresponding anticolor (complementary color) charges. Each quark has one of the three color charges and each antiquark has one of the three anticolor charges. Just as a mix of red, green, and blue light yields white light, in a baryon a combination of "red," "green," and "blue" color charges is color neutral, and in an antibaryon "antired," "antigreen," and "antiblue" is also color neutral. Mesons are color neutral because they carry combinations such as "red" and "antired."

  Because gluon-emission and -absorption always changes color, and -in addition - color is a conserved quantity - gluons can be thought of as carrying a color and an anticolor charge. Since there are nine possible color-anticolor combinations we might expect nine different gluon charges, but the mathematics works out such that there are only eight combinations. Unfortunately, there is no intuitive explanation for this result.

Important Disclaimer:

"Color charge" has nothing to do with the visible colors, it is just a convenient naming convention for a mathematical system physicists developed to explain their observations about quarks in hadrons.

Color charge (Wikipedia)
In particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics (QCD). Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD, there are many technical differences. The "color" of quarks and gluons is completely unrelated to visual perception of color.[1] Rather, it is a name for a property that has almost no manifestation at distances above the size of an atomic nucleus. The term color was chosen because the abstract property to which it refers has three aspects, which are analogized to the three primary colors of red, green, and blue.[2] By comparison, the electromagnetic charge has a single aspect, which takes the values positive or negative.
Shortly after the existence of quarks was first proposed in 1964, Oscar W. Greenberg introduced the notion of color charge to explain how quarks could coexist inside some hadrons in otherwise identical quantum states without violating the Pauli exclusion principle. The theory of quantum chromodynamics has been under development since the 1970s and constitutes an important component of the Standard Model of particle physics.











A hadron with 3 quarks (red, green, blue) before a color change

Red quark emits a red-antigreen gluon

Green quark has absorbed the red-antigreen gluon and is now red; color is conserved


An animation of the interaction inside a neutron

 Color-charged particles cannot be found individually. For this reason, the color-charged quarks are confined in groups (hadrons) with other quarks. These composites are color neutral.
The development of the Standard Model's theory of the strong interactions reflected evidence that quarks combine only into baryons (three quark objects), and mesons (quark-antiquark objects), but not, for example, four-quark objects. Now we understand that only baryons (three different colors) and mesons (color and anticolor) are color-neutral. Particles such as ud or uddd that cannot be combined into color-neutral states are never observed.

Color confinement (Wikipedia)
Color confinement, often simply called confinement, is the physics phenomenon that color charged particles (such as quarks) cannot be isolated singularly, and therefore cannot be directly observed.[1] Quarks, by default, clump together to form groups, or hadrons. The two types of hadrons are the mesons (one quark, one antiquark) and the baryons (three quarks). The constituent quarks in a group cannot be separated from their parent hadron, and this is why quarks can never be studied or observed in any more direct way than at a hadron level.[2]

 The color force favors confinement because at a certain range it is more energetically favorable to create a quark-antiquark pair than to continue to elongate the color flux tube. This is analoguous to the behavior of an elongated rubber-band.

 An animation of color confinement. Energy is supplied to the quarks, and the gluon tube elongates, till it reaches a point where it "snaps" and forms a quark-antiquark pair.

Color-Force Field

The quarks in a given hadron madly exchange gluons. For this reason, physicists talk about the color-force field which consists of the gluons holding the bunch of quarks together.
If one of the quarks in a given hadron is pulled away from its neighbors, the color-force field "stretches" between that quark and its neighbors. In so doing, more and more energy is added to the color-force field as the quarks are pulled apart. At some point, it is energetically cheaper for the color-force field to "snap" into a new quark-antiquark pair. In so doing, energy is conserved because the energy of the color-force field is converted into the mass of the new quarks, and the color-force field can "relax" back to an unstretched state.

 
















Quarks cannot exist individually because the color force increases as they are pulled apart.


 

- Quarks emit gluons - 
Color charge is always conserved. When a quark emits or absorbs a gluon, that quark's color must change in order to conserve color charge. For example, suppose a red quark changes into a blue quark and emits a red/antiblue gluon (the image below illustrates antiblue as yellow). The net color is still red. This is because - after the emission of the gluon - the blue color of the quark cancels with the antiblue color of the gluon. The remaining color then is the red color of the gluon.
Quarks emit and absorb gluons very frequently within a hadron, so there is no way to observe the color of an individual quark. Within a hadron, though, the color of the two quarks exchanging a gluon will change in a way that keeps the bound system in a color-neutral state.

A quark and an antiquark (red color) are glued together (green color) to form a meson (result of a lattice QCD simulation by M. Cardoso et al.

 - Residual strong force - 
So now we know that the strong force binds quarks together because quarks have color charge. But that still does not explain what holds the nucleus together, since positive protons repel each other with electromagnetic force, and protons and neutrons are color-neutral.

So what holds the nucleus together? Huh?

The answer is that, in short, they don't call it the strong force for nothing. The strong force between the quarks in one proton and the quarks in another proton is strong enough to overwhelm the repulsive electromagnetic force.













This is called the residual strong interaction, and it is what "glues" the nucleus together.

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WEAK
There are six kinds of quarks and six kinds of leptons. But all the stable matter of the universe appears to be made of just the two least-massive quarks (up quark and down quark), the least-massive charged lepton (the electron), and the neutrinos.

Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. When fundamental particles decay, it is very strange: we observe the particle vanishing and being replaced by two or more different particles. Although the total of mass and energy is conserved, some of the original particle's mass is converted into kinetic energy, and the resulting particles always have less mass than the original particle that decayed.
The only matter around us that is stable is made up of the smallest quarks and leptons, which cannot decay any further.
 


When a quark or lepton changes type (a muon changing to an electron, for instance) it is said to change flavor. All flavor changes are due to the weak interaction.
Each of the quarks has a different "flavor," which is just the term physicists use to distinguish between the six types of quarks. For instance, the flavor of an up quark is simply "up." Charged weak interactions can change the flavor of a particle! And only charged weak interactions can do this. Weak interactions which involve the neutral Z particle cannot change a particle's flavor. Leptons also have a "flavor." In addition, they have electron number, muon number, and tau number, as discussed earlier. While lepton flavor is changed by weak interactions, the process conserved electron, muon, and tau numbers.
The carrier particles of the weak interactions are the W+, W-, and the Z particles. The W's are electrically charged and the Z is neutral.
The Standard Model has united electromagnetic interactions and weak interactions into one unified interaction called electroweak.

 Weak interaction (Wikipedia)
Weak interaction (often called the weak force or sometimes the weak nuclear force), is one of the four fundamental forces of nature, alongside the strong nuclear force, electromagnetism, and gravity. It is responsible for the radioactive decay of subatomic particles and initiates the process known as hydrogen fusion in stars. Weak interactions affect all known fermions; that is, particles whose spin (a property of all particles) is a half-integer.
In the Standard Model of particle physics the weak interaction is theorised as being caused by the exchange (i.e., emission or absorption) of W and Z bosons; and because it is a consequence of the emission (or absorption) of bosons it is a non-contact force. The best known effect of this emission is beta decay, a form of radioactivity. The Z and W bosons are much heavier than protons or neutrons and it is the heaviness that accounts for the very short range of the weak interaction. It is termed weak because its typical field strength is several orders of magnitude less than that of both electromagnetism and the strong nuclear force. Most particles will decay by a weak interaction over time. It has one unique property – namely quark flavour changing – that does not occur in any other interaction. In addition, it also breaks parity-symmetry and CP-symmetry. Quark flavour changing allows for quarks to swap their 'flavour', one of six, for another.
The weak force was originally described, in the 1930s, by Fermi's theory of a contact four-fermion interaction: which is to say, a force with no range (i.e., entirely dependent on physical contact[1]). However, it is now best described as a field, having range, albeit a very short range. In 1968, the electromagnetic force and the weak interaction were unified, when they were shown to be two aspects of a single force, now termed the electro-weak force.
Weak interactions are most noticeable when particles undergo beta decay and in the production of deuterium and then helium from hydrogen that powers the sun's thermonuclear process. Such decay also makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14. It can also create radioluminescence, commonly used in tritium illumination, and in the related field of betavoltaics.[2]

Left-handed fermions in the Standard Model.[12]
Generation 1 Generation 2 Generation 3
Fermion Symbol Weak
isospin
Fermion Symbol Weak
isospin
Fermion Symbol Weak
isospin
Electron e^-\, -1/2\, Muon \mu^-\, -1/2\, Tau \tau^-\, -1/2\,
Electron neutrino \nu_e\, +1/2\, Muon neutrino \nu_\mu\, +1/2\, Tau neutrino \nu_\tau\, +1/2\,
Up quark u\, +1/2\, Charm quark c\, +1/2\, Top quark t\, +1/2\,
Down quark d\, -1/2\, Strange quark s\, -1/2\, Bottom quark b\, -1/2\,
All left-handed antiparticles have weak isospin of 0.
Right-handed antiparticles have the opposite weak isospin.
 Weak isospin:

















































.
Weak isospin (T3) is a property (quantum number) of all particles, which governs how particles interact in the weak interaction. Weak isospin is to the weak interaction what electric charge is to the electromagnetism, and what color charge is to strong interaction. Elementary particles that are fermions have weak isospin values of ±12.
 
A diagram plotting mass against charge for the six quarks of the standard model, and depicting the various decay routes due to the weak interaction and some indication of their likelihood.

 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

The Feynman diagram for beta-minus decay of a neutron into a proton, electron and electron anti-neutrino, via an intermediate heavy W boson


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The Standard Model - What holds it together? - Electroweak - 
 Unified ElectroWeak Theory





In the Standard Model the weak and the electromagnetic interactions have been combined into a unified electroweak theory.
 Physicists had long believed that weak forces were closely related to electromagnetic forces.
Eventually they discovered that at very short distances (about 10-18 meters) the strength of the weak interaction is comparable to that of the electromagnetic. On the other hand, at thirty times that distance (3x10-17 m) the strength of the weak interaction is 1/10,000th than that of the electromagnetic interaction. At distances typical for quarks in a proton or neutron (10-15 m) the force is even tinier.
Physicists concluded that, in fact, the weak and electromagnetic forces have essentially equal strengths. This is because the strength of the interaction depends strongly on both the mass of the force carrier and the distance of the interaction. The difference between their observed strengths is due to the huge difference in mass between the W and Z particles, which are very massive, and the photon, which has no mass as far as we know.
Electro-Weak force (Wikipedia)

In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 100 GeV, they would merge into a single electroweak force. Thus if the universe is hot enough (approximately 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.
In addition, the gravity force carrier particle has not been found. Such a particle, however, is predicted to exist and may someday be found: the graviton.
Fortunately, the effects of gravity are extremely tiny in most particle physics situations compared to the other three interactions, so theory and experiment can be compared without including gravity in the calculations. Thus, the Standard Model works without explaining gravity.

(I still don't get it.)

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