The curvature of the chiral pseudo-critical line from lattice QCD

The study of the temperature - baryon chemical potential T−μB phase diagram of strongly interacting matter is being performed both experimentally and by theoretical means. The comparison between the experimental chemical freeze-out line and the crossover line, corresponding to chiral symmetry restoration, is one of the main issues. At present it is not possible to perform lattice simulations at real μB because of the sign problem. In order to circumvent this issue, we make use of analytic continuation from an imaginary chemical potential: this approach makes it possible to obtain reliable predictions for small real μB. By using a state-of-the-art discretization, we study the phase diagram of strongly interacting matter at the physical point for purely imaginary baryon chemical potential and zero strange quark chemical potential. We locate the pseudocritical line by computing two observables related to chiral symmetry, namely the chiral condensate and the chiral susceptibility. We then perform a continuum limit extrapolation with Nt=6,8,10 and 12 lattices, obtaining our final estimate for the curvature of the pseudocritical line κ=0.0135(20). Our study includes a thorough analysis of the systematics involved in the definition of Tc(μB), and of the effect of a nonzero μs.