The curvature of the freeze-out line in heavy ion collisions

Understand

We calculate the mean and variance of net-baryon number and net-electric charge distributions from Quantum Chromodynamics (QCD) using a next-to-leading order Taylor expansion in terms of temperature and chemical potentials.

• We compare these expansions with experimental data from STAR and PHENIX, determine the freeze-out temperature in the limit of vanishing baryon chemical potential, and, for the first time, constrain the curvature of the freeze-out line through a direct comparison between experimental data on net-charge fluctuations and a QCD calculation.
• We obtain a bound on the curvature coefficient, kappa_2^f < 0.011, that is compatible with lattice QCD results on the curvature of the QCD transition line.