Color superconductivity and chiral symmetry restoration at non-zero baryondensity and temperature

We explore the phase diagram of strongly interacting matter as a function o f temperature and baryon number density, using a class of models for two-fl avor QCD in which the interaction between quarks is modelled by that induce d by instantons. Our treatment allows us to investigate the possible simult aneous formation of condensates in the conventional quark-anti-quark channe l (breaking chiral symmetry) and in a quark-quark channel leading to color superconductivity: the spontaneous breaking of color symmetry via the forma tion of quark Cooper pairs. At low temperatures, chiral symmetry restoratio n occurs via a first-order transition between a phase with low (or zero) ba ryon density and a high density color superconducting phase. We find color superconductivity in the high density phase for temperatures less than of o rder tens to 100 MeV, and find coexisting [qq] and [(q) over bar q] condens ates in this phase in the presence of a current quark mass, At high tempera tures, the chiral phase transition is second order in the chiral limit and is a smooth crossover for non-zero current quark mass. A tricritical point separates the first-order transition at high densities from the second-orde r transition at high temperatures. In the presence of a current quark mass this tricritical point becomes a second-order phase transition with Ising m odel exponents, suggesting that a long correlation length may develop in he avy ion collisions in which the phase transition is traversed at the approp riate density, (C) 1999 Elsevier Science B.V.