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

TL;DR

This paper uses QCD calculations and experimental data to analyze fluctuations in conserved charges, constraining the curvature of the freeze-out line in heavy ion collisions, and providing new bounds consistent with lattice QCD results.

Contribution

It introduces a novel method to constrain the freeze-out line curvature directly from experimental charge fluctuation data using QCD calculations.

Findings

Bound on the freeze-out line curvature coefficient: kappa_2^f < 0.011

Results are compatible with lattice QCD predictions

First direct constraint on freeze-out line curvature from experimental data

Abstract

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.