fourquark

Fourquark, also known as tetraquark, is a hadron that contains four valence quarks. In the conventional quark model, hadrons are either mesons made of a quark and an antiquark or baryons made of three quarks; fourquark states are exotic hadrons that extend this classification. A fourquark state is necessarily a color-singlet, typically described as either a tightly bound diquark–antidiquark pair (qq)–(q̄q̄) in a color antitriplet and triplet combination, or as a loosely bound molecular state of two color-singlet mesons.

Fourquark states can carry various flavors, including hidden heavy flavors such as cc̄ plus light quarks (u, d, s), or open flavors with four distinct quarks. They may have a range of quantum numbers and can appear as narrow resonances or broader structures near thresholds.

The idea of fourquark states dates to the early development of the quark model, with ideas popularized by R. L. Jaffe in the 1970s and later by others. Experimental candidates have accumulated since the early 2000s, most notably the X(3872) discovered in 2003, which does not fit a conventional charmonium assignment and is widely interpreted as a fourquark or meson-molecule state. Other charged charmonium-like states such as the Z_c(3900) and Z_c(4020) provide strong evidence for non-quarkonia configurations because their electric charge requires four quarks. The interpretation of these states—whether as compact tetraquarks or meson–meson molecules—remains under investigation, with lattice QCD and effective field theory studies contributing to the discussion.

Fourquark states remain a key area of hadron spectroscopy, offering insight into quark confinement, color interactions, and the spectrum of strong-interaction bound states.