quarksantiquarks

Quark-antiquark pairs are bound states formed by a quark and its corresponding antiquark under the strong interaction. Quarks are elementary fermions that carry color charge and come in six flavors: up, down, charm, strange, top, and bottom. Antiquarks carry the corresponding anticolor and opposite quantum numbers. When a quark and an antiquark combine, they can form a color-neutral system known as a meson, one type of hadron.

Mesons have baryon number B = 0 and are bosons when their total spin is an integer (typically 0 or 1 for ground states). Because of color confinement, free quarks are not observed; quark-antiquark pairs exist only within color-singlet hadrons or in more complex multi-quark states. The flavor content of the pair determines the meson’s properties, while quantum numbers such as total angular momentum J, parity P, and charge conjugation C (collectively J^PC) characterize the state.

Common examples include light mesons such as the pions (π^+, π^0, π^−), formed from up and down quarks and their antiquarks, and the kaons (K^+, K^0, etc.), which involve strange quarks. Heavy quarkonia are bound states of a heavy quark with its own antiquark, such as charmonium (c c̄) and bottomonium (b b̄), with well-known states like J/ψ and Υ. These states serve as clean probes of quantum chromodynamics because they exhibit relatively simple, well-defined quantum numbers.

In experiments, quark-antiquark pairs are produced in high-energy collisions and rapidly hadronize into mesons, or they can annihilate into photons or gluons, contributing to various decay channels. Studying their spectra and decays helps test QCD, flavor physics, and the dynamics of confinement.