quark: a standalone construct

Murray Gell-Mann's 1964 paper A schematic model of baryons and mesons, and independently George Zweig's 1964 CERN preprint on an SU(3) model of strong-interaction symmetry, proposed that the strongly interacting particles called hadrons are built from a small set of more basic constituents, which Gell-Mann named quarks. In the modern Standard Model, supported experimentally by the deep inelastic electron-proton scattering of the late 1960s that revealed point-like objects inside the proton, a quark is an elementary spin one-half particle carrying a fractional electric charge of either plus two-thirds or minus one-third of the proton's charge, and it comes in six varieties called flavors: up, down, strange, charm, bottom, and top. Quarks also carry the strong-force property called color, and they are never observed in isolation but only bound together, three quarks forming a baryon such as the proton and a quark with an antiquark forming a meson. What makes the quark a landmark is its scope: it is one of the two families of elementary matter particles in the Standard Model, and together with the leptons it accounts for all known composite matter. As a separate note, the same spelling also names a German fresh cheese, a food word rather than a scientific term.[1][2][3][4]

Unit

An elementary matter particle (a spin one-half fermion), one of the six quark flavors of the Standard Model

Recovery means

Not applicable

Measured by

Inferred from hadron spectroscopy and from deep inelastic scattering and jet production in high-energy collisions; individual flavors identified by their mass, charge, and characteristic decay signatures