Heavyquarkonia

Heavy quarkonia are mesons formed by a heavy quark and its antiquark. The most studied examples are charmonium (a charm quark and an anticharm quark) and bottomonium (a bottom quark and an antibottom quark). Their large quark masses imply that the quarks move non-relativistically inside the bound state, so their spectra can be described by potential models and nonrelativistic QCD, with lattice QCD providing first-principles calculations.

The spectrum features a rich set of states, including S-wave ground states such as J/psi (charmonium) and Upsilon(1S) (bottomonium), as well as excited S-wave, P-wave and higher orbital angular momentum states (e.g., psi(2S), Upsilon(2S), chi_cJ, chi_bJ). Charmonium masses lie around 3 GeV, bottomonium around 9–10 GeV. Hyperfine and fine-structure splittings (such as J/psi–eta_c and Upsilon–eta_b) probe the spin-dependent part of the QCD interaction. The top quark is too short-lived to form bound quarkonia, so toponium does not exist under normal conditions.

Experimentally, heavy quarkonia are produced in electron–positron collisions and in hadron colliders, among other environments. They decay predominantly through annihilation into gluons or photons, with important electromagnetic channels to lepton pairs (e+e−, μ+μ−) that yield clean signals. Their relatively narrow widths and well-separated spectra make them ideal probes of quantum chromodynamics.

Heavy quarkonia serve as key laboratories for testing QCD in the non-relativistic regime, constraining lattice QCD calculations, determining decay constants and strong-coupling effects, and providing benchmarks for quarkonium production mechanisms. The discovery of J/psi in 1974 marked a turning point in hadron physics and the acceptance of the charm quark.