tetraquark

A tetraquark is a hadron composed of four valence quarks, typically two quarks and two antiquarks, unlike ordinary mesons (quark-antiquark) and baryons (three quarks). In quantum chromodynamics, tetraquarks are exotic hadrons bound by the strong interaction and must form a color singlet.

Two main pictures compete in describing their structure. A compact tetraquark is often treated as a bound diquark–antidiquark system [qq][anti-q anti-q], with the diquark in a color anti-triplet and the anti-diquark in a color triplet. Alternatively, a hadronic molecule is a loosely bound system of two color-singlet mesons near a decay threshold. Both pictures can yield similar experimental signatures, and the true nature of many candidates remains under study.

Several candidates have been reported. X(3872), discovered in 2003, is neutral with quantum numbers J^PC = 1++, lying near the D0 D*0 threshold. Charged states such as Z_c(3900) and the Z_b(10610) and Z_b(10650) indicate four-quark content in the charm and bottom sectors. In 2021, LHCb reported a narrow doubly charmed tetraquark candidate, T_cc^+, consistent with a ccud configuration. Interpretations of these states as tetraquarks or meson molecules continue to be debated.

The study of tetraquarks informs quantum chromodynamics by testing diquark correlations, color dynamics, and the line between compact multiquark states and hadronic molecules. Lattice QCD and phenomenological models are used to predict spectra, decays, and production rates, while experiments search for resonant structures in heavy-quarkonium channels and near-threshold meson pairs.

In summary, tetraquarks are a key part of the ongoing exploration of exotic hadrons and multiquark dynamics in QCD.