Phase diagram evolution at finite coupling in strong coupling lattice QCD

We investigate the chiral phase transition in the strong coupling lattice QCD at finite temperature and density with finite coupling effects. We adopt one species of staggered fermion, and develop an analytic formulation based on strong coupling and cluster expansions. We derive the effective potential as a function of two order parameters, the chiral condensate sigma and the quark number density rho(q), in a self-consistent treatment of the next-to-leading order (NLO) effective action terms. NLO contributions lead to modifications of quark mass, chemical potential, and the quark wave function renormalization factor. While the ratio mu(c)(T = 0)/T-c(mu = 0) is too small in the strong coupling limit, it is found to increase as beta = 2N(c)/g(2) increases. The critical point is found to move in the lower T direction as beta increases. Since the vector interaction induced by rho(q) is shown to grow as beta, the present trend is consistent with the results in Nambu-Jona-Lasinio models. The interplay between two order parameters leads to the existence of partially chiral restored matter, where effective chemical potential is automatically adjusted to the quark excitation energy.