Quarks

Quarks

The basic constituent particles of which elementary particles are understood to be composed. Theoretical models built on the quark concept have been very successful in understanding and predicting many phenomena in the physics of elementary particles.

The study of the elastic scattering of electrons on protons demonstrated that the proton has a finite form factor, that is, a finite radial extent of its electric charge and magnetic moment distributions. It was plausible that the charge cloud which constitutes the proton is a probability distribution of some smaller, perhaps pointlike constituents, just as the charge cloud of an atom was learned to be the probability distribution of electrons. Subsequent high-energy, deep inelastic scattering experiments of electrons on protons, leading to meson production, revealed form factors corresponding to pointlike constituents of the proton. These proton constituents, first referred to as partons, are now understood to include the constituent quarks of the proton.

Properties of quarks

Flavor	Mass†, GeV/c2	Electric charge‡	Baryon number	Spin	Isotopic spin	Strangeness	Charm
u	0.0015–0.004	+2/3	+1/3	1/2	1/2	0	0
d	0.004–0.008	-1/3	+1/3	1/2	1/2	0	0
c	1.15–1.35	+2/3	+1/3	1/2	0	0	+1
s	0.080–0.130	-1/3	+1/3	1/2	0	-1	0
t	174.3±5.1	+2/3	+1/3	1/2	0	0	0
b	4.1–4.4	-1/3	+1/3	1/2	0	0	0
†As the mass of baryons composed of quarks is strongly influenced by the gluons binding the quarks, and as free quarks are not observed, the masses are theoretical estimates. ‡Charge is in units of the magnitude of the charge of an electron, 1.6 × 10-19 coulomb. Spin is in units of Planck's constant divided by 2&pgr;, written as ℏ. The top quark mass is deduced from experimental measurements of its decay dynamics.

These high-energy collisions also produced an abundance of resonance states, equivalent to short-lived particles. The spectroscopy of these hadronic states revealed an order and symmetry among the observed hadrons that could be interpreted in terms of representations of the SU(3) symmetry group. This in turn is interpreted as a consequence of the grouping of elementary constituents of fractional electric charge in pairs and triplets to form the observed particles. The general features of the quark model of hadrons have withstood the tests of time, and the static properties of hadrons are consistent with predictions of this model. See Symmetry laws (physics), Unitary symmetry

Thus, the proton and neutron are not fundamental constituents of matter, but each is composed of three quarks, very much as the nuclei of ³H and ³He are made of protons and neutrons, and the molecules of NO₂ and N₂O are made of oxygen and nitrogen atoms.

There are two kinds (or “flavors”) of quarks of very low mass of which the proton, neutron, and pions are composed, and a third, more massive quark which is a constituent of “strange” particles, including the K mesons and hyperons such as the &Lgr;⁰. These are known as the up quark (u), the down quark (d), and the strange quark (s). Baryons are composed of three quarks, for example the proton (uud), neutron (udd), &Lgr;⁰ (uds), and &Xgr;^- (dss). Antiparticles such as antiprotons are formed by the antiquarks of those forming the particle, for example, the antiproton (). Mesons are composed of a quark-antiquark pair, such as the &pgr;⁺(u), &pgr;^-(d), K⁺(u), and K^-(s). See Baryon, Hyperon, Meson, Strange particles

The quantum numbers of quarks are added to give the quantum numbers of the elementary particle which they form on combination. The unit of electrical charge of a quark is +2/3 or -1/3 of the charge on a proton (1.6 × 10^-19 coulomb), and the baryon number of each quark is +1/3 (see table). The charge, baryon number, and so forth, of each antiquark are just the negative of that for each quark.

During the 1970s, experiments at electron-positron colliders and proton accelerators detected a relatively long-lived (that is, very narrow, in energy) resonant state of about 3.1 GeV total energy. This was interpreted as evidence for a new quark, the charm (c) quark, produced as a quark-antiquark resonance analogous to the &phgr;. The discovery of this J/&psgr; resonance was followed by the observation and study of meson systems, now labeled D mesons, containing a single c or quark (paired with an antiquark of another flavor), as well as baryon states containing these quarks. See Charm, J/psi particle, Particle accelerator

A few years later, experiments with higher-energy proton beams, studying the spectra of muon-antimuon pairs at the Fermi National Accelerator Laboratory, discovered a more massive, narrow resonant state at about 9.4 GeV, which was labeled the &Ugr; (upsilon). This was interpreted as evidence for a more massive quark, the b (bottom) quark. Subsequent experiments at proton and electron accelerators confirmed the existence of the b quark and also observed a corresponding family of meson resonant states, now referred to as B mesons.

During the 1990s, experiments observing collisions of protons and antiprotons at an energy of 1.8 TeV in the center of mass established the existence of the t (top) quark, primarily through analysis of its decay to a B meson and a W intermediate vector boson. The t mass of 174.3 ± 5.1 GeV/c² (about the mass of a tungsten atom) is so great that its weak decay through this channel is very fast, and mesonic states of the t and quark (analogous to the &Ugr;, the J/&psgr;, and the &phgr;) are not observed, although the observed t's are from the production of pairs. See Intermediate vector boson

Quarks are understood to have a spin of 1/2; that is, their intrinsic angular momentum is ℏ/2 (where ℏ is Planck's constant h divided by 2&pgr;), just as for the electron and muon. A problem arose when the structure of observed baryons required two or, in some cases, three quarks of the same flavor in the same quantum state, a situation forbidden for spin-1/2 particles by the Pauli exclusion principle. In order to accommodate this contradiction, a new quantum variable, arbitrarily labeled color, was introduced; the idea is that each quark is red, green, or blue (and the antiquarks, antired, and so forth). The color quantum number then breaks the degeneracy and allows up to three quarks of the same flavor to occupy a single quantum state. Confirmation of the color concept has been obtained from experiments with electron-positron storage rings, and the theory of quantum chromodynamics (QCD), based on this concept, has been developed. According to quantum chromodynamics, hadrons must be colorless; for example, baryons must consist of a red, a green, and a blue quark, and mesons of a quark-antiquark pair of the same color (for example, a red quark and an anti-red antiquark). See Color (quantum mechanics), Exclusion principle, Spin (quantum mechanics)

The field quanta of quantum chromodynamics are gluons, massless, spin-1 quanta which interact with quarks. This is very analogous to the manner in which photons, the quanta of electromagnetic interaction, interact with particles containing electric charge and are responsible for electromagnetic forces. The QCD theory is part of the now widely accepted standard model of elementary particle interactions, together with the electroweak theory. Experiments have increasingly confirmed details of the standard model to the extent that most physicists are confident that it is fundamentally correct. See Electroweak interaction, Gluons, Quantum chromodynamics, Standard model

There are three sets, or “generations,” of quarks and leptons. Each generation contains a charged lepton (electron, muon, or tau lepton); a correspoding neutrino; a charge -1/3 quark color triad; and a charge +2/3 quark triad. See Lepton, Neutrino

Quarks and the theory of quantum chromodynamics are now firmly established as cornerstones of the standard model of elementary particles (together with the electroweak theory, charged leptons, neutrinos, and so forth). However, unanswered questions remain.

The advanced string and M theories have the property of supersymmetry, which demands that every spin-1/2 quark and lepton must have a partner with integral spin. As of 2004, no experimental evidence for any of these supersymmetric (SUSY) particles had been found. See Superstring theory, Supersymmetry

Contemporary theories also predict that there exists one or more massive particles of integral spin, the Higgs paticles, responsible for the rest masses of the quarks and charged leptons. Again, the lack of evidence suggests that the Higgs particles must also have a rest mass of over 100 GeV/c², if they exist. See Higgs boson

Quarks may be permanently stable against decay via the weak interaction; however, it is also possible that quarks spontaneously decay to leptons. Intensive searches for the decay of the proton (into a neutral pion and a positron, for example) have been negative, setting a lower limit of over 10³² years for the proton lifetime. However, the apparent asymmetry of the universe between matter and antimatter (there is, at present, no evidence for primordial antimatter) suggests that antiprotons, for example, may spontaneously decay (or transform) more readily. See Antimatter, Proton

Some theories have postulated that quarks are composed of smaller constituents, just as other objects that were originally believed to be fundamental subsequently were found to have internal structure. So far, all observations are compatible with the quarks as point objects, like the electron.