QCD Transition line up to μ=400 MeV from finite volume lattice simulations

A standard way to compute the QCD crossover line in the temperature ( T ) - baryochemical potential ( μ B ) plane with lattice techniques is to extract it from the chiral condensate or from the peak position of the chiral susceptibility. However, these quantities suffer from not-negligible volume effects, as shown in [1]. Here we draw the QCD crossover line from the peak position of an alternative observable, the static quark entropy ( S Q ( T, μ B )), based on the renormalized Polyakov loop, that was shown to have a peak in the vicinity of the chiral transition temperature in [2]. The advantage is that it has smaller volume effects. That allow us to extrapolate S Q ( T, μ B ) by means of a Taylor expansion to eighth order in μ B (NNNLO) on a 16 × 8 lattice. We use high-statistics simulations with 2+1 4HEX staggered fermions at physical quark masses. The results are computed along the strangeness neutral line. We are able to draw the phase diagram up to μ B ≈ 400 MeV, finding a rough agreement with phenomenological estimates of the freeze-out curve in relativistic heavy ion collisions. We see that the width of the crossover increases at higher densities, disfavoring the existence of a critical endpoint in the explored range.