Charmonium, exotic hadrons and hadron structure

To celebrate the 50th anniversary of the discovery of the J/ψ, the first charmonium state observed, I start with a brief review of major progresses on the QCD inspired quark potential model originated from charmonium spectrum.Then I show the importance of unquenching dynamics, multiquark components and exotic multiquark states for understanding hadron structure and hadron spectrscopy. The J/ψ and charmonium-like states have played an important role in this aspect.

💡 Research Summary

The paper commemorates the 50‑year anniversary of the J/ψ discovery by tracing the evolution of quark‑potential models from their inception to the modern unquenched, multiquark frameworks that are now essential for describing hadron spectroscopy. It begins with a concise historical overview: the 1960s “particle zoo” led to Gell‑Mann’s and Zweig’s quark hypothesis, and the 1974 observation of the narrow J/ψ resonance provided the first clean realization of a heavy‑quark (c c̄) bound state. Because the charm quark mass (~1.5 GeV) makes the system non‑relativistic, the Cornell potential V(r)=−(4/3)αs/r+κr, embodying one‑gluon exchange at short distances and a linearly rising confinement term at large distances, successfully reproduced the charmonium and bottomonium spectra, establishing a QCD‑inspired dynamical picture of hadrons.

The author then discusses why this simple “Coulomb + linear” potential fails for light quarks. Chiral symmetry, spontaneously broken in the QCD vacuum, generates Goldstone bosons (π, K, η) whose exchange supplies a long‑range attractive force. Hidden local gauge symmetry introduces vector mesons (ρ, ω, K*, ϕ) as gauge bosons, providing intermediate‑range interactions. The modern chiral quark (or constituent‑quark) model therefore combines a confining potential, short‑range one‑gluon exchange, and medium/long‑range meson exchanges, achieving a unified description of ground‑state mesons and baryons across all flavors.

Despite this success, the model cannot account for many excited states because it neglects “unquenching” – the creation of quark‑antiquark pairs that couple the bare q q̄ (or qqq) configurations to meson‑meson (or meson‑baryon) channels. Empirical evidence for sizable multiquark components is presented: deep‑inelastic scattering and Drell‑Yan experiments reveal a pronounced (\bar d-\bar u) asymmetry (~0.12), indicating a u u d d (\bar d) component of >12 % in the proton; strange form‑factor measurements imply a non‑negligible u u d s (\bar s) admixture. Meson‑cloud models (e.g., n π⁺) and diquark‑diquark‑antiquark configurations (