Critical line of 2+1 flavor QCD: Toward the continuum limit

We determine the continuum limit of the curvature of the pseudocritical line of QCD with nf=2+1 staggered fermions at nonzero temperature and quark density. We perform Monte Carlo simulations at imaginary baryon chemical potentials with Highly Improved Staggered Quarks (HISQ), adopting the HISQ/tree action discretization as implemented in the code by the MILC Collaboration. Couplings are adjusted so as to move on a line of constant physics, as determined in Ref. [1], with the strange quark mass ms fixed at its physical value and a light-to-strange mass ratio ml/ms=1/20. The chemical potential is set at the same value for the three quark species, μl=μs≡μ. We attempt an extrapolation to the continuum using the results on lattices with temporal size up to Lt=12. Our estimate for the continuum value of the curvature κ at zero baryon density, κ=0.020(4), is compared with recent lattice results and with experimental determinations of the freeze-out curve.