Quarkonium

Quarkonium refers to mesons formed by a heavy quark and its antiquark bound by the strong interaction. The term is typically applied to charm–anticharm (charmonium) and bottom–antibottom (bottomonium) states. Because the heavy quark mass is large, these systems can be described by nonrelativistic quantum mechanics, and their spectra are approximately hydrogen-like with several bound states below open-flavor thresholds. Top quark pairs would in principle form toponium, but the top quark decays weakly too quickly to hadronize, so toponium has not been observed.

Notable charmonium states include the J/psi (1S) and psi(2S) triplet states, the eta_c singlet, and their excitations. In bottomonium, the Upsilon family (Upsilon(1S), Upsilon(2S), Upsilon(3S), etc.) and the eta_b singlet appear alongside various excited states. Each system has spin-singlet and spin-triplet members, leading to fine and hyperfine splittings.

Theoretical descriptions rely on QCD, often in effective nonrelativistic forms. Potential models, such as the Cornell potential combining a short-range coulomb term with linear confinement, reproduce qualitative spectroscopy. More fundamental approaches include lattice QCD and nonrelativistic QCD.

Decays and transitions provide experimental access to internal structure. Quarkonia annihilate into gluons or lepton pairs, and radiative transitions between states occur via electric dipole (E1) or magnetic dipole (M1) processes. Ground and low-lying states typically have relatively narrow widths.

Quarkonium states are produced in e+e− annihilation, hadron colliders, and dedicated flavor facilities. They serve as important laboratories for testing quantum chromodynamics in the nonperturbative regime, calibrating quark masses, and validating lattice calculations.