Multiquark bound states and resonances

TL;DR

The paper surveys how multiquark binding could arise from chromoelectric (CE) or chromomagnetic (CM) interactions, or from quark interchange that yields a coupled-channel hadron–hadron dynamics. In the quark model, a naive color-octet exchange yields no bound states, so binding requires mass asymmetry or spin-dependent and three-body forces; CM-based schemes (C1-C3) have had some success in predicting exotics, while CE interactions can bind $QQ\bar{u}\bar{d}$ with favorable mass ratios, evidenced by the near-threshold $T_{cc}^+$. Fully-heavy systems such as $cc\bar{c}\bar{c}$ are not bound in simple models, though resonances may appear, and lattice QCD results sometimes challenge the constituent-potential picture, suggesting missing dynamics. A quark-exchange viewpoint is developed as a complementary picture, with a concrete four-quark calculation showing how channel coupling between color-singlet configurations can generate an effective interaction along the inter-hadron coordinate and potentially produce near-threshold structures such as in $J/\psi J/\psi$ channels. The authors call for quantitative dynamical calculations, spin-aware antisymmetrization, and broader lattice and experimental cross-checks, particularly for hidden-charm/beauty pentaquarks and double-heavy tetraquarks.

Abstract

We review the chromoelectric and chromomagnetic mechanisms that tentatively lead to stable or metastable multiquark configurations. An alternative interpretation of the dynamics is the quark interchange between hadrons, as illustrated in the case of the fully-charm systems.