Chiral phase transition in a covariant nonlocal NJL model

The properties of the chiral phase transition at finite temperature and che mical potential are investigated within a nonlocal covariant extension of t he Nambu-Jona-Lasinio model based on a separable quark-quark interaction. W e consider both the situation in which the Minkowski quark propagator has p oles at real energies and the case where only complex poles appear. In the literature, the latter has been proposed as a realization of confinement. I n both eases, the behaviour of the physical quantities as functions of T an d mu is found to be quite similar. In particular, for low values of T the c hiral transition is always of first order and, for finite quark masses, at certain "end point" the transition turns into a smooth crossover. In the ch iral limit, this "ead point" becomes a "tricritical" point. Our predictions for the position of these points are similar, although somewhat smaller, t han previous estimates. Finally, the relation between the deconfining trans ition and chiral restoration is also discussed. (C) 2001 Published by Elsev ier Science B.V.