Quarkantiquark

Quarkantiquark, usually written quark–antiquark or qq̄, denotes a bound state of a quark and its corresponding antiquark. In the conventional quark model developed in the 1960s, such pairs form mesons, which are color singlets and obey Bose statistics because they carry integer spin. The strong interaction binding these constituents is described by quantum chromodynamics, with gluons mediating the force and color confinement ensuring that observed hadrons are color neutral.

Quarkonia, the heavy-quark–antiquark systems, include charmonium (charm–anticharm, c c̄) and bottomonium (bottom–antibottom, b b̄). Light mesons include pions and kaons. States are organized by total spin S, orbital angular momentum L, and total angular momentum J, with parity and charge-conjugation quantum numbers J^PC. The spectrum exhibits spin-spin, spin-orbit, and tensor splittings and is described by potential models and lattice QCD in addition to quark-model classifications.

qq̄ mesons are produced in high-energy collisions and in decays of heavier particles; they decay via strong and electromagnetic interactions to lighter hadrons. Measurements of their masses, widths, and decays test quantum chromodynamics, constrain quark masses, and inform the determination of the strong coupling constant.

Although the conventional picture treats mesons as qq̄ states, some resonances may have more complex structures, such as tetraquarks or molecular states, and the interpretation can depend on experimental data.