Diquark Bose Condensates in High Density Matter and Instantons

Instantons lead to strong correlations between up and down quarks with spin zero and antisymmetric color wave functions. In cold and dense matter, $n_b>n_c\simeq 1{\mathrm{fm}}^{-3\mathrm{}}$ and $T<\mathrm{}{T}_c\sim 50$ MeV, these pairs Bose condense, replacing the usual $〈\overline{q}q〉$ condensate and restoring chiral symmetry. At high density, the ground state is a color superconductor in which diquarks play the role of Cooper pairs. An interesting toy model is provided by QCD with two colors: it has a particle-antiparticle symmetry which relates $〈\overline{q}q〉$ and $〈\mathrm{qq}〉$ condensates.