Current quark mass effects on the chiral phase transition of QCD in the improved ladder approximation

Current quark mass effects on the chiral phase transition of QCD are studied in the improved ladder approximation. The infrared behavior of the gluon propagator is modified in terms of an effective running coupling. The analysis is based on a composite operator formalism and a variational approach. We use the Schwinger-Dyson equation to give a "normalization condition" for the Cornwall-Jackiw-Tomboulis effective potential and to isolate the ultraviolet divergence that appears in the expression for the quark-antiquark condensate. We study the current quark mass effects on the order parameter at zero temperature and density. We then calculate the effective potential at finite, temperature and density and investigate the current quark mass effects on the chiral phase transition. We find a smooth crossover for T>0, μ=0 and a first-order phase transition for μ>0, T=0. Critical exponents are also studied and our model gives the classical mean-field values. We also study the temperature dependence of the masses of scalar and pseudoscalar bosons. A critical end point in the T-μ plane is found at T∼100 MeV, μ∼300 MeV.