Quark

Quarks are among the most fundamental particles known, smaller and simpler than the protons and neutrons they build. There are six types, called flavors: up, down, charm, strange, top, and bottom, and they combine in specific groups to form larger particles called hadrons.

A proton is made of two up quarks and one down quark, while a neutron is made of two down quarks and one up quark. Quarks are never found alone in nature, they are always bound together by the strong force, a phenomenon called confinement.

Quarks were proposed independently by Murray Gell-Mann and George Zweig in 1964 to explain patterns in the growing zoo of particles being discovered at the time. Their existence was later confirmed directly in high energy scattering experiments.

Direct evidence for quarks came from deep inelastic scattering experiments in the late 1960s at the Stanford Linear Accelerator Center, where firing high energy electrons at protons revealed that the electrons occasionally bounced off something small and hard inside, exactly as if the proton contained tiny, pointlike constituents rather than being a uniform blob of matter. At extremely high temperatures, like those recreated in heavy ion collisions at facilities such as CERN’s Large Hadron Collider, quarks and gluons can briefly break free of confinement altogether and form a superheated state called quark-gluon plasma, thought to be the same state the entire universe was in for the first few millionths of a second after the Big Bang.

Quark masses vary enormously, the lightest, the up quark, weighs a small fraction of a proton’s mass, while the heaviest, the top quark, weighs about as much as an entire gold atom, roughly 184 times heavier than a proton. The top quark is also so short lived, decaying before it has time to bind with other quarks at all, that physicists study its properties through the debris of its decay rather than the particle itself.