Quarks are one of the two basic constituents of Matter in the Standard Model of Particle physics. (The others are leptons.) Antiparticles of quarks are called antiquarks. Quarks and antiquarks are the only fundamental particles which interact through all four of the fundamental forces.
The single most important property of quarks is called Confinement. This is the experimental fact that individual quarks are not seen — they are always confined inside hadrons, subatomic particles like protons, neutrons, and mesons. This fundamental property is expected to follow from the modern theory of strong interactions, called Quantum chromodynamics (QCD). Although there is no mathematical derivation of confinement in QCD, it is easy to show using Lattice gauge theory. [image]
The composite particles made of quarks and antiquarks are the hadrons. These include the mesons which get their quantum numbers from a quark and an antiquark, and the baryons, which get theirs from three quarks. The quarks (and antiquarks) which impart quantum numbers to hadrons are called valence quarks. Apart from these, any hadron may contain an indefinite number of virtual quarks, antiquarks and gluons which together contribute nothing to their quantum numbers. Such virtual quarks are called sea quarks.
| Flavour in particle physics |
Flavour quantum numbers
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Each quark is assigned a Baryon number, B = 1/3, and a vanishing Lepton number L = 0. They have fractional electric Charge, Q, either Q = +2/3 or Q = −1/3. The former are called up-type quarks, the latter, down-type quarks. Each quark is assigned a Weak isospin: Tz = +1/2 for an up-type quark and Tz = −1/2 for a down-type quark. Each doublet of weak isospin defines a generation of quarks. There are three generations, and hence six flavours of quarks — the up-type quarks have flavours u, c and t, the down-type quark flavours are d, s, b.
The number of generations of quarks and leptons are equal in the standard model. The number of generations of leptons is strongly constrained by experiments at the LEP in CERN and by observations of the abundance of Helium in the universe. Precision measurement of the lifetime of the Z boson at LEP constrains the number of generations to be three. Astronomical observations of helium abundance give consistent results. Results of direct searches for a fourth generation give limits on the mass of the lightest possible fourth generation quark. The most stringent limit comes from analysis of results from the Tevatron collider at Fermilab, and shows that the mass of a fourth-generation quark must be greater than 190 GeV.
Each flavour defines a quantum number which is conserved under the strong interactions, but not the weak interactions. The magnitude of flavour changing in the weak interaction is encoded into a structure called the CKM matrix. This also encodes the CP violation allowed in the Standard Model. The flavour quantum numbers are described in detail in the article on flavour.
The only other coloured particle is the Gluon, which is the gauge boson of QCD. Like all other non-Abelian gauge theories (and unlike Quantum electrodynamics) the gauge bosons interact with one another by the same force that affects the quarks.
Colour is a gauged SU(3) symmetry. Quarks are placed in the Fundamental representation, 3, and hence come in three colors. Gluons are placed in the Adjoint representation, 8, and hence come in eight varieties. For more on this, see the article on Color charge.
| m u ––– m d | = 0.56 and | m s ––– m d | = 20.1. |
The top quark is sufficiently heavy that Perturbative QCD can be used to determine its mass. Before its discovery in 1995, the best theoretical estimates of the top quark mass are obtained from global analysis of precision tests of the Standard Model. The top quark, however, is unique amongst quarks in that it decays before having a chance to hadronize. Thus, its mass can be directly measured from the resulting decay products. This can only be done at the Tevatron which is the only Particle accelerator energetic enough to produce top quarks in abundance.
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| Flavour | Name | Generation | Charge | Mass (MeV) |
| Iz=+1/2, Tz=+1/2 | Up (u) | 1 | +2/3 | 1.5 to 4.0 |
| Iz=−1/2, Tz=−1/2 | Down (d) | 1 | −1/3 | 4 to 8 |
| S=−1, Tz=−1/2 | Strange (s) | 2 | −1/3 | 80 to 130 |
| C=1, Tz=+1/2 | Charm (c) | 2 | +2/3 | 1150 to 1350 |
| B′=−1, Tz=−1/2 | Bottom (b) | 3 | −1/3 | 4100 to 4400 |
| T=1, Tz=+1/2 | Top (t) | 3 | +2/3 | 172700 ± 2900 |
In this scheme the lightest mesons (spin-0) and baryons (spin-½) are grouped together into octets, 8, of flavour symmetry. A classification of the spin-3/2 baryons into the representation 10 yielded a prediction of a new particle, Ω−, the discovery of which in 1964 led to wide acceptance of the model. The missing representation 3 was identified with quarks.
This scheme was called the eightfold way by Gell-Mann, a clever conflation of the octets of the model with the eightfold way of Buddhism. He also invented the name quark and attributed it to the sentence “Three quarks for Muster Mark” in James Joyce's Finnegans Wake. (James Gleick's book Genius asserts that this was an after-the-fact attribution, and notes that physicists pronounced quark to rhyme with cork, not Mark.) The negative results of quark search experiments caused Gell-Mann to hold that quarks were mathematical fiction.
Analysis of certain properties of high energy reactions of hadrons led Richard Feynman to postulate substructures of hadrons, which he called partons (since they form part of hadrons). A scaling of Deep Inelastic Scattering cross sections derived from current algebra by James Bjorken received an explanation in terms of partons. When Bjorken scaling was verified in an experiment in 1969, it was immediately realized that partons and quarks could be the same thing. With the proof of Asymptotic freedom in QCD in 1973 by David Gross, Frank Wilczek and David Politzer the connection was firmly established.
The charm quark was postulated by Sheldon Glashow, Iliopoulos and Maiani in 1973 to prevent unphysical flavour changes in weak decays which would otherwise occur in the Standard Model. The discovery in 1975 of the meson which came to be called the J/ψ led to the recognition that it was made of a charm quark and its antiquark.
The existence of a third generation of quarks was predicted by Kobayashi and Maskawa who realized that the observed violation of CP symmetry by neutral kaons could not be accommodated into the Standard Model with two generations of quarks. The bottom quark was discovered in 1980 and the top quark in 1996 at the Tevatron collider in Fermilab.
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| Particles in Physics - elementary particles | |
| Fermions: Quarks | Leptons | |
| Quarks: Up | Down | Strange | Charm | Bottom | Top | |
| Leptons: Electron | Muon | Tau | Neutrinos | |
| Gauge bosons: Photon | W and Z bosons | Gluons | |
| Not yet observed: Higgs boson | Graviton | Other hypothetical particles | |