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Restoration of chiral symmetry in the large-N_c limit

We study the large-N_c behavior of the critical temperature T_c for chiral symmetry restoration in the framework of the Nambu-Jona-Lasinio (NJL) model and the linear σ-model. While in the NJL case T_c scales as N_c^0 and is, as expected, of the same order as Λ_QCD (just as the deconfinement phase transition), in the σ-model the scaling behavior reads T_c \propto N_c^1/2. We investigate the origin of the different scaling behavior and present two improvements of the σ-model: (i) a simple, phenomenologically motivated temperature dependence of the parameters and (ii) the coupling to the Polyakov loop. Both approaches lead to the scaling T_c \propto N_c^0.