Tetraquark

A hadron whose valence content consists of four quarks, usually two quarks and two antiquarks, bound by the strong interaction. Its internal structure may be compact or resemble two loosely bound mesons.

Quark: An elementary fermion that carries color charge and combines with other quarks into hadrons. Tetraquarks are built from quarks and antiquarks, whose flavors determine their quantum numbers.
Meson molecule: A loosely bound state of two mesons, held together by residual strong forces. It is a hadronic-scale alternative to a compact tetraquark structure.
X(3872): A narrow charmonium-like state discovered in 2003, with a mass close to the neutral D-meson pair threshold. Its threshold proximity makes it a leading candidate for a meson-molecule or mixed structure.
Meson: A hadron with one valence quark and one valence antiquark. A tetraquark has two additional valence constituents beyond the conventional meson pattern.
Antiquark: The antiparticle of a quark, carrying opposite electric charge and color charge. Most observed tetraquark candidates combine two quarks with two antiquarks.
Diquark: A correlated pair of quarks that can act as a subsystem inside a hadron. Some tetraquark models organize the constituents into a diquark and an antidiquark.
Z(4430): A charged charmonium-like resonance observed in the decay of B mesons. Its electric charge and hidden charm require at least four valence quarks.
Baryon: A hadron conventionally composed of three valence quarks. Its standard three-quark structure contrasts with the four constituents of a tetraquark.
Color charge: The quantum-chromodynamic charge carried by quarks and gluons, with red, green, and blue labels. Color combinations must make the tetraquark a color singlet overall.
Color singlet: A quantum state with no net color charge, permitted as an isolated hadron. Four-quark color couplings can produce an overall color singlet in more than one way.

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