Extracting freeze-out conditions in beam energy scan via functional QCD

TL;DR

This paper uses functional QCD to determine freeze-out conditions in heavy ion collisions by comparing theoretical susceptibilities with experimental cumulants, predicting a peak in kurtosis near the QCD critical point.

Contribution

It introduces a self-consistent method to extract freeze-out parameters from functional QCD and predicts kurtosis behavior, including a peak near the critical end point.

Findings

Quantitative agreement with experimental data for susceptibilities.

Identification of a kurtosis peak near 5 GeV collision energy.

Evidence supporting the existence of the QCD critical end point.

Abstract

Fluctuations of conserved charges provide a link between high quality theoretical results and precision measurements of heavy ion collisions. We compare results for ratios of the lowest order baryon number susceptibilities from functional QCD approaches to proton number cumulants extracted from experiments. We find that they meet at a specific temperature and chemical potential for each collision energy. This is indicative of the respective freeze-out point. From this self-consistent determination of the freeze-out parameters we extract a prediction for the kurtosis on the freeze-out line. We find quantitative agreement with experimental data where available, despite comparing apples (baryons) with oranges (protons). At a collision energy around 5 GeV, our kurtosis exhibits a peak structure indicative of the critical end point of QCD.