tetraquarks

Tetraquarks are a class of hadrons that contain four valence quarks, typically arranged as two quarks and two antiquarks. Unlike conventional mesons (quark–antiquark) or baryons (three quarks), tetraquarks represent an exotic configuration allowed by quantum chromodynamics. The quark content can vary, with possibilities including charm, bottom, or light quarks, and can give rise to charged states that cannot be explained by simple quark–antiquark pairs.

The internal structure of tetraquarks is a topic of active research. Two common pictures are a compact diquark–antidiquark arrangement, where a tightly bound pair of quarks behaves as a single unit, and a hadronic molecule, where two mesons loosely bound by residual strong forces form the state. The two pictures have different implications for properties such as mass, decay modes, and production mechanisms. Heavy-quark systems, in particular, offer a valuable testing ground for theoretical models.

Historical and experimental context shows a progression of candidates. The X(3872), discovered in 2003, was one of the first strong tetraquark candidates and is often interpreted as a cc̄-based state with possible molecular components. The charged Zc states discovered in 2013 and later experiments provided compelling evidence for four-quark configurations, since their charge requires more content than a simple quark–antiquark pair. In the bottom sector, Zb states observed in the early 2010s added to the growing list of tetraquark candidates. A notable recent result is the observation by LHCb in 2021 of a doubly charmed tetraquark, Tcc+, with content ccū d̄, near open-ch charm thresholds.

Status remains evolving: while several states are widely discussed as tetraquark candidates, their exact internal structure—compact four-quark states versus hadronic molecules—remains debated. Ongoing experiments at LHCb, Belle II, BESIII, and advances in lattice QCD continue to refine understanding of these exotic hadrons.