Quark

The quark is a fundamental fermion and a constituent of matter in the Standard Model of particle physics. Quarks are the only particles that experience all four fundamental forces — strong, weak, electromagnetic, and gravitational — and they are the building blocks of composite particles called hadrons, including the protons and neutrons that form atomic nuclei. Quarks carry fractional electric charge — either +2/3 or −1/3 of the elementary charge — and they possess a property called color charge that makes them the sole participants in the strong interaction mediated by gluons.

The quark model was proposed independently by Murray Gell-Mann and George Zweig in 1964 to explain the proliferating zoo of hadrons discovered in particle accelerator experiments. Gell-Mann originally proposed three quark flavors — up, down, and strange — as the fundamental constituents of the known hadrons. The model explained why certain particle combinations appeared and others did not: hadrons could only form as color-neutral combinations, either as baryons (three quarks, one of each color) or mesons (a quark and an antiquark). The model's predictive power was spectacular: the existence of the Ω⁻ baryon, composed of three strange quarks, was predicted before its experimental discovery in 1964.

Properties

Quarks are spin-1/2 fermions, meaning they obey the Pauli exclusion principle and have half-integer intrinsic angular momentum. They transform as triplets under the SU(3) color group of quantum chromodynamics (QCD), and they also carry weak isospin and hypercharge, making them participants in the electroweak interaction.

There are six known quark flavors, organized into three generations:

• First generation: up (u) and down (d) — the lightest quarks, constituents of protons (uud) and neutrons (udd)
• Second generation: charm (c) and strange (s) — the strange quark explains the long lifetime of kaons; the charm quark was predicted to suppress flavor-changing neutral currents
• Third generation: top (t) and bottom (b) — the top quark is the heaviest known fundamental particle (~173 GeV); the bottom quark appears in B-meson physics

Each quark flavor has a corresponding antiquark with opposite electric charge and color. The top quark is unique in that it decays before it can hadronize, making it the only quark whose properties can be studied as a free particle — albeit fleetingly.

Quarks and Hadrons

The color confinement mechanism of QCD ensures that quarks are never observed in isolation. All observed hadrons are color-neutral composites. Baryons consist of three quarks, one of each color (red, green, blue), and include the proton, neutron, and a spectrum of heavier resonances. Mesons consist of a quark-antiquark pair, with the color of the quark canceling the anticolor of the antiquark. The hadron spectrum — the masses, spins, and quantum numbers of the hundreds of known hadrons — is determined by the dynamics of confined quarks and gluons, and it remains one of the most active frontiers in QCD.

The quark masses are not directly observable, because isolated quarks do not exist. What is measured are the constituent