hiddencharm

Hidden charm is a term used in hadron spectroscopy to describe states that contain a charm quark and its antiquark, such that the charm quantum number cancels and the net charm of the hadron is zero. This distinguishes them from open-charm hadrons, like D mesons and charmed baryons, which carry net charm. Hidden-charm states can be conventional charmonium, where the charm and anti-charm form a bound system, or exotic hadrons that include additional light quarks or gluons in the same cc̄ configuration.

Notable examples include the charmonium family, such as J/ψ, ψ(2S), and χcJ states, which are well described as cc̄ bound states. In addition, several exotic hadrons with hidden charm have been observed, including X(3872), Y(4260), and Zc(3900), which challenge simple quark-model descriptions. Hidden-charm pentaquarks, such as Pc(4380)+ and Pc(4450)+ reported by the LHCb collaboration in 2015, are interpreted as states containing a cc̄ pair with additional light quarks, and remain a topic of active study.

Production and decays of hidden-charm states occur in a variety of processes, including electron-positron annihilation, B-meson decays, and high-energy hadron collisions. They often decay to final states containing J/ψ or other charmonium states plus light hadrons, or to dilepton pairs, providing clean experimental signatures.

The study of hidden charm tests quantum chromodynamics and hadron structure, informing potential models, lattice QCD calculations, and models of multiquark dynamics. It continues to be an active area of inquiry, with ongoing discoveries and refinements in both conventional and exotic cc̄-containing states.