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Phase diagram in an external magnetic field beyond a mean-field approximation

The phase structure of the Polyakov loop-extended chiral quark-meson model is explored in a nonperturbative approach, beyond a mean-field approximation, in the presence of a magnetic field. We show that by including meson fluctuations one cannot resolve the qualitative discrepancy on the dependence of the crossover transition temperature in a non-zero magnetic field between effective model predictions and recent lattice results (arXiv:1111.4956).We compute the curvature of the crossover line in the T - μ_B plane at a non-zero magnetic field and show that the curvature increases with increasing magnetic field. On the basis of QCD inequalities, we also argue that, at least in the large Nc limit, a chiral critical end point and, consequently, a change from crossover to a first-order chiral phase transition are excluded at zero baryon chemical potential and non-zero magnetic field.