Measurements of Proton High Order Cumulants in 3 GeV Au+Au Collisions and Implications for the QCD Critical Point

TL;DR

This study measures proton cumulants in 3 GeV Au+Au collisions to explore fluctuations related to the QCD critical point, finding results consistent with baryon conservation and hadronic interactions, suggesting the critical region occurs at higher energies.

Contribution

First measurement of proton cumulants at 3 GeV in fixed-target collisions, providing new insights into fluctuations and the QCD phase diagram at low energies.

Findings

Proton cumulant ratio C4/C2 is less than unity, indicating suppression.

Results align with baryon number conservation effects.

Data suggest the QCD critical point is at energies above 3 GeV.

Abstract

We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity ($y$) and transverse momentum ($p_{\rm T}$) within $-0.5 < y<0$ and $0.4 < p_{\rm T} <2.0 $ GeV/$c$. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be $C_4/C_2=-0.85 \pm 0.09 ~(\rm stat.) \pm 0.82 ~(\rm syst.)$, which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our $C_4/C_2$ in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in $C_4/C_2$ is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.