charmoniummolecule

Charmoniummolecule refers to a hypothetical hadronic bound state in quantum chromodynamics in which two charmonium units are held together by residual strong interactions. Charmonium themselves are bound states of a charm quark and an anti-charm quark; a charmoniummolecule would be a molecular-like arrangement of two such quarkonium mesons, or, in some models, a bound state formed by a charmonium and a second charmonium-like component. The idea is to extend the concept of hadronic molecules, where colorless hadrons bind through meson exchange or other residual forces.

Predicted properties of a charmoniummolecule include a mass near the sum of the masses of its constituents minus a small binding energy, and quantum numbers determined by the angular momenta and spins of the constituent charmonia. Binding energies are expected to be small compared with the overall mass, so such states would often be narrow resonances and decay primarily into lighter charmonia and light hadrons.

The formation mechanism typically involves meson exchange forces between heavy quarkonia or short-range gluonic interactions in an effective field theory framework. Lattice QCD and QCD-inspired models have explored possible attraction in channels with suitable quantum numbers, but results depend on the chosen interaction model and parameters.

Experimental evidence for a charmoniummolecule is not established. Searches have been conducted in high-energy collisions and B-meson decays, with some analyses claiming hints but no definitive confirmation. Ongoing programs at facilities such as LHCb, Belle II, and BESIII aim to clarify whether stable or near-threshold charmoniummolecules exist.

See also: hadronic molecule, charmonium, exotic hadron, X(3872).