QCD at Finite Baryon Density: Chiral Symmetry Restoration and Color Superconductivity

Abstract

We use a variational procedure to study finite density QCD in an approximation in which the interaction between quarks is modelled by that induced by instantons. We find that uniform states with conventional chiral symmetry breaking have negative pressure with respect to empty space at all but the lowest densities, and are therefore unstable. This is a precisely defined phenomenon which motivates the basic picture of hadrons assumed in the MIT bag model, with nucleons as droplets of chiral symmetry restored phase. This suggests that the phase transition to a chirally symmetric phase occurs by percolation of preexisting droplets of the symmetric phase, and in these proceedings we expand upon our previous presentation of this observation. At all densities high enough that the chirally symmetric phase fills space, color symmetry is broken by the formation of a <qq> condensate of quark Cooper pairs. A plausible ordering scheme leads to a substantial gap in a Lorentz scalar channel involving quarks of two colors.