The phase diagram of the quark-meson model

Within the proper-time renormalization group approach, the chiral phase diagram of a two-flavor quark-meson model is studied. In the chiral limit, the location of the tricritical point which is linked to a Gaussian fixed point, is determined. For quark chemical potentials smaller than the tricritical one the second-order phase transition belongs to the O(4) universality class. For temperatures below the tricritical one we find initially a weak first-order phase transition which is commonly seen in model studies and also in recent lattice simulations. In addition, below temperatures of T-bar 17 MeV we find two phase transitions. The chiral restoration transition is initially also of first-order but turns into a second-order transition again. This leads to the possibility that there may be a 'second tricritical' point in the QCD phase diagram in the chiral limit