QCD-like theories at finite baryon density

We study QCD-like theories with pseudoreal fermions at finite baryon densit y. Such theories include two-color QCD with quarks in the fundamental repre sentation of the color group as well as any-color QCD with quarks in the ad joint color representation. In all such theories the lightest baryons are d iquarks, At zero chemical potential mu they are, together with the pseudosc alar mesons, the Goldstone modes of a spontaneously broken enlarged chiral symmetry group. Using symmetry principles, we derive the low-energy effecti ve Lagrangian for these particles. We find that a second order phase transi tion occurs at a value of mu equal to half the mass of the Goldstone modes. For values of mu beyond this point the scalar diquarks Bose condense and t he diquark condensate is nonzero. We calculate the dependence of the chiral condensate, the diquark condensate, the baryon charge density, and the mas ses of the diquark and pseudoscalar excitations on mu at finite bare quark mass and scaler diquark source. The relevance of our results to lattice QCD calculations and to real three-color QCD at finite baryon density is discu ssed. (C) 2000 Elsevier Science B.V. All rights reserved.