QQbar

qqbar denotes a quark–antiquark pair, a color-singlet bound state that forms mesons in the quark model. The notation uses q for a quark flavor (up, down, strange, charm, bottom, and, in principle, top) and qbar for the corresponding antiquark. Because of color confinement, isolated quarks do not exist; qqbar pairs hadronize into color-neutral mesons.

In spectroscopy, qqbar states explain the meson spectrum. Heavy-quarkonia such as charmonium (ccbar) and bottomonium (bbbar) consist of a heavy quark and its antiquark bound by the strong interaction; their resonances include J/psi (ccbar) and Upsilon (bbbar). Light qqbar states form the light meson nonets (pions, kaons, eta, etc.). The quantum numbers of a qqbar state depend on the spin S and orbital angular momentum L, with total J and parity–charge conjugation determined accordingly. In practice, many light mesons mix with glue or multi-quark configurations, so simple qqbar pictures are approximations.

Production and decay: qqbar pairs are produced in high-energy processes such as gluon splitting g->qqbar in hadronic jets or via electroweak annihilation to virtual photons or Z bosons in e+e− collisions. They hadronize into mesons and subsequently decay through strong, electromagnetic, or weak interactions. Top quarks, due to their short lifetime, decay before hadronization, so true toponium bound states do not form.

Observables: The presence of qqbar states is inferred from meson resonances in experiments, with masses and widths encoding the binding dynamics and the QCD potential. The study of qqbar systems provides insights into confinement, the strong force, and heavy-quark effective theories.