mesonmolecule

A mesonmolecule is a hadronic bound state composed of two or more mesons held together by residual strong interactions, rather than a conventional quark-antiquark meson. In this concept, binding is typically weak and the overall size can be larger than that of ordinary mesons. Meson molecules are usually discussed as near-threshold phenomena, appearing close to the combined mass of their constituent mesons.

The binding mechanism for meson molecules often involves meson exchange forces, such as pion exchange, or other residual interactions that arise when color-neutral hadrons interact. Theoretical descriptions employ effective field theories, phenomenological potential models, and, where possible, lattice QCD calculations. Because the binding is weak and the states lie near thresholds, their properties—such as masses, decays, and line shapes—are highly sensitive to the dynamics of the constituent mesons and to isospin-breaking effects.

Several candidates have been proposed or studied as meson molecules. The X(3872) is among the most discussed, widely interpreted as a D0 and D̄*0 meson molecule near the D0 D̄*0 threshold. Other proposed meson-molecule states include candidates near the KK̄ threshold, such as f0(980) and a0(980), and various near-threshold states involving charm or bottom mesons (for example, near D D̄* or B B̄* thresholds). In many cases, alternate interpretations exist, including compact multiquark configurations, so the molecular interpretation remains an active topic of experimental and theoretical study.

Understanding meson molecules informs the broader study of hadron structure and the ways in which QCD can organize matter beyond conventional quark-model mesons and baryons.