charmonium

Charmonium is a family of mesons consisting of a charm quark and its antiquark (c c̄). It is a subset of quarkonium and serves as a key system for studying quantum chromodynamics (QCD) in the nonrelativistic regime because the charm mass is large compared with ΛQCD. The spectrum exhibits a rich pattern of radial and orbital excitations.

The first charmonium states, notably the J/psi, were discovered in 1974 in electron–positron annihilation experiments, providing decisive evidence for the charm quark. The J/psi is the ground-state 1S charmonium with J^PC = 1^−−; the pseudoscalar partner eta_c is 1S0. The first excited S-wave state is psi(2S). P-wave states chi_c0, chi_c1, chi_c2 (0++, 1++, 2++) and the singlet h_c (1+−) complete the low-lying multiplets. The lowest-lying D-wave state psi(3770) is commonly interpreted as a 1D1 state, and higher excitations continue to be observed.

Production and decays: charmonia are produced in e+e− annihilation at resonances, in B decays, and in high-energy hadron collisions. They decay primarily by annihilation into gluons with hadronic final states, and via radiative transitions to lower-lying charmonia; leptonic decays to e+e− or μ+μ− provide clean experimental signatures. The measured patterns of masses, widths, and transition rates test potential models and lattice QCD calculations.

Significance: as a heavy-quark bound state, charmonium provides a laboratory for testing QCD, tuning quark-mass determinations, and exploring quark confinement and nonperturbative dynamics.

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