ccbar

ccbar refers to a charm quark and its antiquark pair. In particle physics, ccbar systems form bound states known as charmonium, a class of mesons sometimes called quarkonia. The most famous member is the J/ψ meson, a vector state with quantum numbers J^PC = 1^--, discovered in 1974 and instrumental in establishing the existence of charm quarks. The charmonium spectrum also includes the ψ(2S), the pseudoscalar ηc, and the χc0, χc1, and χc2 triplet, corresponding to different orbital and spin configurations. Charmonium states have masses in the range of roughly 2 to 3.5–4 GeV, with the J/ψ mass about 3.097 GeV.

Production and decays: ccbar states are produced in high-energy processes such as electron–positron annihilation, hadronic collisions, and photoproduction. They can decay electromagnetically to lepton pairs (e+e− or μ+μ−) or radiatively to lower charmonium states plus photons, as well as into various hadronic final states. The J/ψ, in particular, exhibits a relatively narrow width, which makes it a clean experimental signal and a useful probe in collider experiments.

Theoretical importance: charmonium is studied as a nonrelativistic bound state of a heavy quark and antiquark, providing a testing ground for quantum chromodynamics (QCD). It informs potential models, effective theories such as NRQCD, and lattice QCD calculations, contributing to broader understanding of quark confinement and hadronization in the strong interaction.