QCD bulk thermodynamics and conserved charge fluctuations with HISQ fermions

TL;DR

This paper reviews recent lattice QCD results on the thermodynamics and conserved charge fluctuations using HISQ fermions, highlighting universal scaling behaviors near the QCD critical point and providing model-independent estimates of freeze-out parameters.

Contribution

It presents new lattice QCD calculations of conserved charge fluctuations with HISQ fermions, demonstrating universal scaling and offering a method to determine freeze-out conditions without model assumptions.

Findings

Higher order cumulants follow universal scaling laws.

Ratios of cumulants are volume independent and comparable to experiments.

Freeze-out parameters can be extracted directly from lattice QCD data.

Abstract

After briefly reviewing recent progress by the HotQCD collaboration in studying the 2+1 flavor QCD equation of state, we will focus on results on fluctuations of conserved charges by the BNL-Bielefeld and HotQCD collaborations. Higher order cumulants of the net-charge distributions are increasingly dominated by a universal scaling behavior, which arises due to a critical point of QCD in the chiral limit. Considering cumulants up to the 6^{th} order, we observe that they generically behave as expected from universal scaling laws, which is quite different from cumulants calculated within the hadron resonance gas model. Taking ratios of these cumulants, we obtain volume independent results that can be compared to the experimental measurements. We will argue that the freeze-out chemical potentials and the freeze-out temperature, usually obtained by a HRG model fit to the measured hadronic yields, can also be obtained in a model independent way from ab-initio lattice QCD calculations by utilizing observables related to conserved charge fluctuations. Further, we will show that the freeze-out strangeness and electric charge chemical potentials can be fixed by imposing strangeness neutrality and isospin asymmetry constraints in the lattice QCD calculations, in order to accommodate conditions met in heavy ion collisions. All results have been obtained with the highly improved staggered quark action (HISQ) and almost physical quark masses on lattices with temporal extent of N_\tau=6,8,10,12.