Threshold Collision Energy of the QCD Phase Diagram Tricritical Endpoint

TL;DR

This paper investigates irregularities in chemical freeze-out data from nuclear collisions, suggesting they indicate two phase transitions in QCD, including a possible tricritical endpoint near 8.8-9.2 GeV.

Contribution

It provides evidence for two distinct phase transitions in the QCD phase diagram and proposes the collision energy range of 8.8-9.2 GeV as near the tricritical endpoint.

Findings

Irregularities at 4.9 GeV and 9.2 GeV indicate phase transitions.

Sharp peaks of baryonic charge density observed.

Collision energy range 8.8-9.2 GeV may be near the QCD tricritical endpoint.

Abstract

Using the most advanced formulation of the hadron resonance gas model we analyze the two sets of irregularities found at chemical freeze-out of central nuclear-nuclear collisions at the center of mass energies 3.8-4.9 GeV and 7.6-9.2 GeV. In addition to previously reported irregularities at the collision energies 4.9 GeV and 9.2 GeV we found sharp peaks of baryonic charge density. Also we analyze the collision energy dependence of the modified Wroblewski factor and the strangeness suppression factor. Based on the thermostatic properties of the mixed phase of a 1-st order phase transition and the ones of the Hagedorn mass spectrum we explain, respectively, the reason of observed chemical equilibration of strangeness at the collision energy 4.9 GeV and above 8.7 GeV. It is argued that the both sets of irregularities possibly evidence for two phase transitions, namely, the 1-st order transition at lower energy range and the 2-nd order transition at higher one. In combination with a recent analysis of the light nuclei number fluctuations we conclude that the center of mass collision energy range 8.8-9.2 GeV may be in the nearest vicinity of the QCD tricritical endpoint. The properties of the phase existing between two phase transitions are revealed and discussed.