Charmonium is a family of bound states of a charm quark and its antiquark, with zero net charm. It differs from open-charm D mesons, which carry a charm quark together with a light antiquark.
The constituent spins and relative orbital angular momentum distinguish states such as the J/psi meson, eta c meson and chi c mesons. Their level separations and radiative transitions test the interaction between heavy quarks. Decay can proceed through annihilation of the charm–anticharm pair, so net charm conservation does not imply that a charmonium state is stable. A valence quark label organizes the spectrum while additional gluonic and quark-pair contributions remain part of the interacting state.
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