QCD phases at high density and instantons

Abstract

The talk is an introduction into diquark condensation phenomena which occur in QCD at high energy density. They are driven by instantons and instanton-antiinstanton pairs (or ``molecules''), which generate attraction in some qq channels. A number of phases is possible, with or without restoration of chiral symmetry: the work is not finished and we do not yet know which take place in real QCD. We also emphasize that specific diquark correlations play a significant role in baryon structure, in particular making that of a nucleon very different of a $\Delta$ (or other member of a decuplet). This ``small $N_c$'' scenario based on comparison to QCD with two colors is contrasted with the ``large $N_c$'' one.