quark

A quark (pronounced /kwɔrk/ or /kwɑrk/) is a type of elementary particle found in nucleons (protons and neutrons) and other subatomic particles. They are a major constituent of matter, along with leptons (such as electrons and neutrinos). In technical terms, quarks are elementary fermions and are subject to the Pauli exclusion principle. They are the only particles in the Standard Model to experience all four fundamental forces, also known as fundamental interactions.[1] In nature, quarks are never found on their own, as isolated, single particles; rather, they are bound together in composite particles named hadrons, [2][3] the most common being protons and neutrons which are the basic building blocks of atomic nuclei. For this reason, much of what is known about quarks has been inferred from observations on the hadrons themselves. Quarks (and antiquarks) are the only known particles whose electric charge comes in fractions of the elementary charge. However this can never be directly observed as hadrons all have integer charge.

There are six different types of quarks, known as flavors: up (symbol: u), down (d), charm (c), strange (s), top (t) and bottom (b).[4] The flavors with the lowest masses, the up quark and the down quark, are generally stable and are very common in the universe. The heavier charm, strange, top and bottom quarks are unstable and rapidly decay; these can only be produced in high energy collisions, such as in particle accelerators and in cosmic rays. Quarks have various properties, such as electric and color charge, spin and mass. For every quark flavor there is a corresponding antiparticle, called antiquark, that differs from the quark only in that some of its properties have the opposite sign.

The quark model was independently proposed by physicists Murray Gell-Mann and George Zweig in 1964.[5] There was little evidence for the physical reality of quarks until 1968, when electron–proton scattering experiments indicated that the electrons were scattering off three point-like constituents inside the proton.[6][7] By 1995, when the top quark was observed at Fermilab, all six flavors had been observed.
The Standard Model is the theoretical framework describing all the currently known elementary particles, plus the Higgs boson (unobserved as of 2008[update]). This model comprises six flavors of quarks, named up, down, charm, strange, top and bottom.[4] The top and bottom flavors are sometimes known as truth and beauty, respectively.[8] In this context, flavor is an arbitrarily chosen term referring to different kinds of particles, and has nothing to do with the everyday experience of flavor. All quarks of the same flavor are identical particles, meaning that all of their properties are the same.

In the Standard Model, particles of matter, including quarks and leptons, are fermions, meaning that their spin quantum number (a property related to their intrinsic angular momentum) is half-integer; as a consequence, they are subject to the Pauli exclusion principle, stating that no two fermions of the same flavor can ever simultaneously occupy the same state. This contrasts with particles that mediate forces—the other particles of the standard model. Such particles are bosons, meaning that they have integer spin; as a consequence, the Pauli exclusion principle does not apply to them.[9] Quarks, unlike leptons, have a color charge, a property causing them to engage in the strong interaction. This interaction is the reason quarks attract each other to form hadrons. In the same way that the electric force is responsible for atoms attracting each other to form molecules, the strong interaction is responsible for protons and neutrons attracting each other to form atomic nuclei. But unlike the electric force which has infinite range, the strong interaction effectively only acts at close distances, of order 10−15 m or less. See nuclear force for more details.

Elementary fermions are grouped into three generations, each comprising two leptons and two quarks. The first generation includes up and down quarks, the second charm and strange quarks, and the third top and bottom quarks. All searches for a fourth generation of quarks and other elementary fermions have failed, and there is strong indirect evidence that more than three generations cannot exist: each generation comprises only one flavor of neutrino, and the existence of a fourth generation would imply values of the lifetime of the Z boson and the abundance of helium-4 in the universe that are at odds with experimental results.[10] Particles in higher generations generally have greater mass and are less stable, tending to decay into lower-generation, less massive particles by means of weak interactions. Only the first-generation up and down quarks occur commonly in nature; heavier quarks can only be created in high-energy collisions, such as in cosmic rays, and quickly decay. As a result, these particles play little part in the universe of today, but likely were much more prominent in an earlier, hotter universe. Most studies conducted on heavier quarks have been performed in artificially created conditions, such as in particle accelerators.[11]

Antiparticles of quarks are called antiquarks, and are denoted by a bar over the letter for the quark, such as u for an up antiquark. As with antimatter in general, antiquarks have the same mass, lifetime and spin as their respective quarks, but the electric charge and other charges have the opposite sign.[12]

Having electric charge, flavor, color charge and mass, quarks are the only known elementary particles that engage in all four fundamental interactions of contemporary physics: electromagnetism, weak interaction, strong interaction and gravitation. Gravitation, however, is usually irrelevant at subatomic scales, and is not described by the Standard Model.

See the table of properties below for a more complete analysis of the six quark flavors' properties.