Diquark condensation at nonzero chemical potential and temperature

SU(2) lattice gauge theory with four flavors of quarks is studied at nonzer o chemical potential mu and temperature T by computer simulation and Effect ive Lagrangian techniques. Simulations are done on 8(4), 8(3) x 4 and 12(3) x 6 lattices and the diquark condensate, chiral order parameter, Wilson li ne, fermion energy and number densities are measured. Simulations at a fixe d, nonzero quark mass provide evidence for a tricritical point in the mu -T plane associated with diquark condensation. For low T, increasing tt takes the system through a line of second order phase transitions to a diquark c ondensed phase. Increasing T at high mu, the system passes through a line o f first order transitions from the diquark phase to the quark-gluon plasma phase. Using Effective Lagrangians we estimate the position of the tricriti cal point and ascribe its existence to trilinear couplings that increase wi th mu and T. (C) 2001 Elsevier Science B.V. All rights reserved.