diquarks

A diquark is a correlated pair of quarks within a hadron. It is not a free particle, but rather an effective, strongly bound subsystem that can influence the structure and dynamics of the parent hadron.

In quantum chromodynamics, diquarks carry color charge and must combine with a third quark to form a color-singlet baryon. The interaction is most attractive when the two quarks are in a color antitriplet state, which supports their tendency to cluster. Diquarks are not observed in isolation; they are used as a theoretical concept to describe internal quark correlations.

Two common low-lying diquark configurations are the scalar diquark and the axial-vector diquark. The scalar diquark has J^P = 0^+ and is flavor antisymmetric (typically the ud pair with isospin I = 0). The axial-vector diquark has J^P = 1^+ and is flavor symmetric (such as uu, dd, or ud with I = 1). These configurations, especially the scalar one, are often discussed as the most tightly bound or most prominent correlations inside baryons.

Diquarks feature prominently in various hadron models, where baryons are treated as a bound state of a diquark and a remaining quark. This approach can simplify descriptions of baryon spectra, magnetic moments, and form factors, and it appears in some pentaquark constructions as a diquark-diquark-antiquark arrangement.

There is no experimental evidence for free diquarks. Lattice QCD and phenomenological studies support the existence of strong diquark correlations in certain channels, while emphasizing that diquarks act as effective degrees of freedom within hadrons rather than independent particles. In dense quark matter, diquark condensation is also discussed in the context of color superconductivity.