quark

Quark is a fundamental constituent of matter in the Standard Model. Quarks are elementary fermions that combine to form hadrons, such as protons and neutrons, which comprise atomic nuclei. They are characterized by flavor, electric charge, spin, and color charge, and they are never observed in isolation due to color confinement.

There are six flavors: up, down, charm, strange, top, and bottom. Up, charm, and top have electric charge +2/3e; down, strange, and bottom have -1/3e. All quarks have spin 1/2 and obey Fermi-Dirac statistics. They carry color charge, coming in red, green, and blue; antiquarks carry corresponding anticolors. Quarks participate in all fundamental interactions, most notably the strong interaction, which is mediated by gluons.

The strong interaction is described by quantum chromodynamics (QCD). Gluons themselves carry color charge, leading to phenomena such as confinement and asymptotic freedom. Quarks also engage in electromagnetic and weak interactions; the weak interaction is responsible for quark flavor-changing decays, such as those of the heavier quarks.

Quarks bind to form hadrons. Baryons are composed of three quarks (for example, the proton uud and the neutron udd). Mesons consist of a quark and an antiquark (such as the pion). The combination must be color-neutral overall. The masses of quarks range from a few MeV for up and down quarks (current quark masses) to about 173 GeV for the top quark; bound-state masses differ due to binding energy and gluonic contributions.

Historically, quarks were proposed in 1964 by Murray Gell-Mann and George Zweig to explain hadron patterns. The term quark comes from James Joyce. Experimental evidence came from deep inelastic scattering at SLAC in 1968, revealing pointlike constituents inside protons. Today, quarks are studied with colliders, lattice QCD, and phenomenological models.