the quark
/ kwork /
Peel a proton like an onion and you never reach a smooth core; you find three restless specks that no experiment has ever pried loose on their own. Those specks are quarks, the constituents of protons, neutrons and the whole zoo of strongly-interacting particles. They answer a question the periodic table could not: what are protons and neutrons actually made of? The name, borrowed by Murray Gell-Mann from a line in Joyce's Finnegans Wake, hides a very concrete idea, a small set of fundamental fermions from which almost all the mass around you is built.
A quark is a spin-1/2 fermion carrying fractional electric charge: the up-type quarks (up, charm, top) have charge +2/3 e and the down-type quarks (down, strange, bottom) have charge -1/3 e, in units of the proton charge e. There are six flavors in three generations, spanning an enormous mass range from the up quark at a few MeV to the top quark near 173 GeV. Crucially every quark also carries one of three color charges (red, green, blue), the source of the strong interaction described by quantum chromodynamics. Because color is confined, quarks combine only into color-neutral bunches: three quarks make a baryon such as the proton (uud), a quark and an antiquark make a meson such as the pion.
Quarks were first inferred as an accounting scheme for the 1960s particle zoo and then seen directly as hard scattering centers inside the proton in deep inelastic electron scattering at SLAC, the 'partons' of Bjorken and Feynman. A common misconception is that a proton is simply three quarks sitting still; in fact those three valence quarks float in a boiling sea of gluons and virtual quark-antiquark pairs, and most of the proton's mass comes not from the quark masses themselves but from the energy of that strong-field binding, via E = m c^2.
The proton is uud (charge 2/3 + 2/3 - 1/3 = 1) and the neutron is udd (charge 2/3 - 1/3 - 1/3 = 0); a single beta decay is, at the quark level, one down quark turning into an up quark, which is why the neutron becomes a proton.
Fractional quark charges add up to the familiar integer charges of the proton and neutron.
The top quark is so heavy that it decays in about 10^-25 s, faster than the time needed to form a hadron, so it is the one quark that never binds into a bound state and is instead studied as a nearly free particle.