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

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 ($p_{\mathrm{T}}$) within $-0.5<y<0$ and $0.4<p_{\mathrm{T}}<2.0$ GeV/$c$. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be $C_4/C_2=-0.85\pm 0.09(stat.)\pm 0.82(syst.)$, 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.