Off-diagonal correlators of conserved charges from lattice QCD and how to relate them to experiment

TL;DR

This paper calculates off-diagonal correlators of conserved charges using lattice QCD, relates them to experimental data through HRG models, and estimates the chemical freeze-out temperature in heavy ion collisions.

Contribution

It provides continuum-extrapolated lattice results for off-diagonal correlators at zero and finite chemical potential, and develops a method to connect these to experimental measurements.

Findings

Estimated chemical freeze-out temperature at 165 MeV.

Demonstrated small dependence of certain ratios on acceptance cuts.

Compared lattice results with STAR experimental data.

Abstract

Like fluctuations, non-diagonal correlators of conserved charges provide a tool for the study of chemical freeze-out in heavy ion collisions. They can be calculated in thermal equilibrium using lattice simulations, and be connected to moments of event-by-event net-particle multiplicity distributions. We calculate them from continuum extrapolated lattice simulations at $\mu_B=0$, and present a finite-$\mu_B$ extrapolation, comparing two different methods. In order to relate the grand canonical observables to the experimentally available net-particle fluctuations and correlations, we perform a Hadron Resonance Gas (HRG) model analysis, which allows us to completely break down the contributions from different hadrons. We then construct suitable hadronic proxies for fluctuations ratios, and study their behavior at finite chemical potentials. We also study the effect of introducing acceptance cuts, and argue that the small dependence of certain ratios on the latter allows for a direct comparison with lattice QCD results, provided that the same cuts are applied to all hadronic species. Finally, we perform a comparison for the constructed quantities for experimentally available measurements from the STAR Collaboration. Thus, we estimate the chemical freeze-out temperature to 165 MeV using a strangeness-related proxy. This is a rather high temperature for the use of the Hadron Resonance Gas, thus, further lattice studies are necessary to provide first principle results at intermediate $\mu_B$.