Diquarks and Density

We describe aspects of the role that diquark correlations play in understanding baryon structure and interactions. The significance of diquarks in that application motivates a study of the possibility that dense hadronic matter may exhibit diquark condensation; i.e., quark-quark pairing promoted by a quark chemical potential. A Gorkov-Nambu-like gap equation is introduced for QCD and analysed for 2-colour QCD (QC2D) and, in two qualitatively different truncations, for QCD itself. Among other interesting features, we illustrate that QC2D with massive fermions undergoes a second-order transition to a superfluid phase when the chemical potential exceeds 0.5 m.pi. In the QCD application we illustrate that the sigma= <bar-q q> .neq. 0 phase, which determines the properties of the mass spectrum at zero temperature and chemical potential, is unstable with respect to the superfluid phase when the chemical potential exceeds approx. 2 sigma, and that at this point the diquark gap is large, approx. 0.5 sigma. The superfluid phase survives to temperatures greater than that expected in the core of compact stars.