The Chiral and Deconfinement Phase Transition in QCD

The phase diagram of strongly interacting matter is one of the most exciting subjects of modern particle physics. While QCD lattice calculations become more and more realistic at vanishing chemical potential, their predictive power at finite chemical potential is still very limited due to the fermion sign problem. Until now lattice calculations cannot provide insights on the existence of a possible QCD critical end point (CEP), where the chiral crossover pass into a first-order phase transition. By means of chiral effective models, such as the linear σ-model (LσM) with three quark flavors, one can access arbitrary regions in the phase diagram [1]. In general, chiral models do not incorporate explicit gluonic degrees of freedom and hence are unable to describe confinement properties of QCD. However, this can be cured by adding an effective Polyakov loop potential and embedding the Polyakov loop to the fermionic part of the LσM. In the framework of the resulting PolyakovQuark-Meson (PQM) model both, the chiral and the deconfinement transitions can be investigated. Several possible choices for the Polyakov loop potential are available [2]. In Fig. 1 the phase diagram is shown for a logarithmic Polyakov loop potential. The model parameters have been adjusted such that both transitions coincide at μ = 0. At some finite chemical potential both transition lines start to deviate and the emergence of a quarkyonic phase becomes possible. The PQM model enables a more realistic description of the equation of state, which is demonstrated in Figs. 2 and