Diquarks: condensation without bound states

We employ a bispinor gap equation to study superfluidity at nonzero chemical potential: µ 6 = 0, in two- and three-colour QCD. The two-colour theory, QC 2 D, is an excellent exemplar: the order of truncation of the quark-quark scattering kernel: K , has no qualitative impact, which allows a straightforward elucidation of the effects of µ when the coupling is strong. In rainbow-ladder truncation, diquark bound states appear in the spectrum of the three-colour theory, a defect that is elim-inated by an improvement of K . The corrected gap equation describes a superfluid phase that is semi-quantitatively similar to that obtained using the rainbow truncation. A model study suggests that the width of the superfluid gap and the transition point in QC 2 D provide reliable quantitative esti-mates of those quantities in QCD.