No abstract

We investigate the chiral phase transition at nonzero temperature T and baryon-chemical potential µ B within the framework of the linear sigma model and the Nambu-Jona-Lasinio model. For small bare quark masses we find in both models a smooth crossover transition for nonzero T and µ B = 0 and a first order transition for T = 0 and nonzero µ B . We calculate explicitly the first order phase transition line and spinodal lines in the (T, µ B ) plane. As expected they all end in a critical point. We find that, in the linear sigma model, the sigma mass goes to zero at the critical point. This is in contrast to the NJL model, where the sigma mass, as defined in the random phase approximation, does not vanish. We also compute the adiabatic lines in the (T, µ B ) plane. Within the models studied here, the critical point does not serve as a "focusing" point in the adiabatic expansion.

We calculate quarkonium spectral functions in a quark-gluon plasma using a potential model based on full QCD lattice calculations of the free energy of a static quark-antiquark pair. We estimate the binding energy and the thermal width of different quarkonium states. The estimated upper limit for the dissociation temperatures is considerably lower than the ones suggested in the recent literature.

We study quarkonium correlators and spectral functions at zero and finite temperature in QCD with only heavy quarks using potential models combined with perturbative QCD. First, we show that this approach can describe the quarkonium correlation function at zero temperature. Using a class of screened potentials based on lattice calculations of the static quark-antiquark free energy we calculate spectral functions at finite temperature. We find that all quarkonium states, with the exception of the 1S bottomonium, dissolve in the deconfined phase at temperatures smaller than 1.5Tc, in contradiction with the conclusions of recent studies. Despite this the temperature dependence of the quarkonium correlation functions calculated on the lattice is well reproduced in our model. We also find that even in the absence of resonances the spectral function at high temperatures is significantly enhanced over the spectral function corresponding to free quark antiquark propagation.