Quarks

Quarks are elementary particles and fundamental constituents of matter in the Standard Model. They combine to form hadrons, such as protons and neutrons, which make up atomic nuclei.

There are six flavors of quarks: up, down, charm, strange, top, and bottom. They have spin 1/2 and electric charges of either +2/3 or -1/3, depending on flavor. Quarks carry a color charge—red, green, or blue—and must combine to color-neutral (singlet) states. Each quark has a baryon number of 1/3, and antiquarks carry −1/3. Quarks occur in three generations: (up, down), (charm, strange), and (top, bottom).

Quarks interact via the strong, weak, and electromagnetic forces. The strong interaction is described by quantum chromodynamics (QCD), a non-Abelian gauge theory based on SU(3) symmetry, with gluons mediating forces between color charges. Quarks also couple to the electroweak force through W and Z bosons and photons, and gravitational interactions are present but negligible at subatomic scales. A defining property of quarks is color confinement: isolated quarks have not been observed, and they exist only within color-neutral hadrons. At very high energies, the strong coupling weakens (asymptotic freedom), allowing quarks to behave almost as free particles.

Hadrons come in two main families: baryons (three quarks) and mesons (a quark and an antiquark). The proton and neutron are baryons; pions are mesons. Quark masses vary widely: up and down quarks are a few MeV, strange about 100 MeV, charm around 1.3 GeV, bottom around 4.2 GeV, and top about 173 GeV; the top quark is exceptionally heavy and decays before forming hadrons.

Quarks were proposed in 1964 by Gell-Mann and Zweig. The name was inspired by James Joyce’s Finnegans Wake. Evidence includes deep inelastic scattering and hadron spectroscopy, with ongoing studies refining our understanding of their properties.