Lattice QCD results for the coefficient c2 appearing in the Taylor expansion of the pressure show that this quantity raises with the temperature towards the Stefan-Boltzmann limit. On the other hand, model approximations predict that when a vector repulsion, parametrized by GV , is present this coefficient reaches a maximum just after Tc and then deviates from the lattice predictions. Recently, this discrepancy has been used as a guide to constrain the (presently unknown) value of GV within the framework of effective models at large-Nc (LN). In the present investigation we show that, due to finite Nc effects, c2 may also develop a maximum even when GV = 0 since a vector repulsive term can be dynamically generated by exchange type of radiative corrections. Here we apply the the Optimized Perturbation Theory (OPT) method to the two flavor Polyakov-NambuJona-Lasinio model (at GV = 0) and compare the results with those furnished by lattice simulations an by the LN approximation at GV = 0 and also at GV = 0. The OPT numerical results for c2 are impressively accurate for T 1.2 Tc but, as expected, predict that this quantity develops a maximum at high-T . After identifying the mathematical origin of this extremum we argue that such a discrepant behavior may naturally arise within these effective quark models (at GV = 0) whenever the first 1/Nc corrections are taken into account. We then interpret this hypothesis as an indication that beyond the large-Nc limit the correct high temperature (perturbative) behavior of c2 will be faithfully described by effective models only if they also mimic the asymptotic freedom phenomenon.

No abstract

We analyze the two flavor version of the Nambu-Jona-Lasinio model with a repulsive vector coupling , at finite temperature and quark chemical potential, in the strong scalar coupling