mesonmeson

Mesonmeson is a term used in hadron spectroscopy to denote a hypothetical bound state formed by two mesons, i.e., a mesonmeson molecule. In this picture, two color-singlet mesons interact via residual strong forces and form a loosely bound system with a total mass near the sum of the constituent masses. The concept is part of the broader idea of hadronic molecules, distinct from conventional quark-antiquark mesons and from compact multi-quark states.

Formation and properties: Binding typically occurs in near-threshold states, often in S-wave configurations; the binding energy is small compared with the constituent masses. The quantum numbers reflect the combination of the mesons’ spins and parities. Bindings may be mediated by meson exchange (such as pions for light flavors) or other residual interactions in quantum chromodynamics. The resulting states are expected to have relatively narrow widths if they lie just below strong decay thresholds.

Theoretical status: Mesonmeson states are studied with effective field theories and lattice quantum chromodynamics, using approaches that incorporate heavy-quark symmetry and chiral dynamics. They provide a natural framework for explaining certain near-threshold resonances that do not fit easily into the traditional quark model.

Experimental status: Several near-threshold candidates have been discussed as mesonmeson molecules. The X(3872) is one of the most studied examples, commonly interpreted as a D0 D*0bar molecule. Other candidates include bottomonium-like charged states near B B* thresholds, which have been interpreted as molecular or tetraquark in nature.

See also: hadronic molecule; meson; tetraquark; exotic hadron; heavy quarkonium.