Definition
A quark is a fundamental particle of the Standard Model that combines with other quarks to form protons, neutrons, and the broader family of hadrons. There are six flavors — up, down, charm, strange, top, bottom — each carrying a fractional electric charge (+2/3 or –1/3) and one of three "color" charges of the strong force.
Quarks were proposed by Gell-Mann and Zweig in 1964 and confirmed in deep inelastic scattering experiments at SLAC in 1968-69.
Why it matters
How it works
Quarks live in three "generations," each containing one up-type (charge +2/3) and one down-type (charge –1/3) quark. The first generation — up and down — makes the stable matter around us. The second (charm, strange) and third (top, bottom) generations are heavier copies, produced in cosmic rays and accelerators and decaying quickly into lighter quarks.
The strong force, described by quantum chromodynamics (QCD), is mediated by eight gluons that couple to color charge. Three colors are usually labeled red, green, and blue (the names are arbitrary — they have nothing to do with visible color). A hadron must be color-neutral, achieved either by three quarks of three different colors (a baryon like the proton or neutron) or by a quark-antiquark pair with matching color-anticolor (a meson like the pion).
Color confinement is the striking feature: pull two quarks apart and the gluon field between them stores increasing energy as a "flux tube," which eventually snaps by creating a new quark-antiquark pair. You end up with two color-neutral hadrons rather than two isolated quarks. This is why the proton's mass — about 938 MeV — is dominated by gluon-field binding energy, while the up and down quarks themselves are only a few MeV each.
The top quark, found at Fermilab in 1995, is by far the heaviest at 173 GeV — roughly the mass of a gold atom in one elementary particle. It decays so fast (about 5 × 10⁻²⁵ s) that it never has time to hadronize.