Diquark

A diquark is a correlated pair of quarks within a hadron that behaves as a single unit in certain theoretical descriptions of strong interactions. Diquarks are not observed as free particles; they arise from the color confinement of quantum chromodynamics (QCD) and are used as an effective degrees of freedom to simplify the treatment of hadron structure.

In QCD, two quarks can combine into a color antitriplet state, which is an attractive channel. Diquarks can occur in different spin and flavor configurations. The two most discussed are the scalar diquark with total spin 0 and the axial-vector diquark with total spin 1. The scalar (often called the good diquark) is typically flavor antisymmetric (for example the ud pair with isospin I=0) and is considered to be the more tightly bound, while the axial-vector (the bad diquark) is flavor symmetric (I=1) and has higher energy. The common picture assigns the diquark a color antitriplet and a roughly S-wave spatial configuration within the parent hadron.

Diquarks play a prominent role in models of baryons, where a quark-diquark picture treats a baryon as a bound state of a diquark and a remaining quark, aiding spectroscopy and decay analyses. They also appear in phenomenological descriptions of multiquark states, such as tetraquarks and pentaquarks, where diquark–antidiquark correlations can simplify the expected structures.

Evidence for diquark correlations comes from lattice QCD studies and various phenomenological analyses, though diquarks remain a theoretical construct rather than independently observable particles. Their applicability depends on the context and the level of approximation used in the modeling.

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