Charged-Particle Multiplicity Dependence of Net-Proton Cumulants in Zr+Zr and Ru+Ru Collisions at $\sqrt{s_{NN}}$ = 200 GeV

TL;DR

This study measures higher-order cumulants of net-proton distributions in Zr+Zr and Ru+Ru collisions at 200 GeV, revealing trends consistent with lattice QCD predictions and suggesting a transition to thermalized QCD matter with increasing multiplicity.

Contribution

First measurements of net-proton cumulant ratios up to sixth order in isobaric collisions at 200 GeV, comparing results with lattice QCD and other collision systems.

Findings

Higher-order cumulant ratios decrease with increasing charged-particle multiplicity.

Isobar collision results align with Au+Au collision trends within uncertainties.

Cumulant ratios approach lattice QCD predictions as multiplicity increases.

Abstract

We present measurements of cumulants of event-by-event net-proton distribution at mid-rapidity and their ratios up to the sixth order as a function of charged-particle multiplicity in Zr+Zr and Ru+Ru(isobars) collisions at a nucleon-nucleon center-of-mass energy ($\sqrt{s_{NN}}$) of 200 GeV. The data are collected from the STAR experiment with a total of two billion events recorded for each collision system. The measurements are compared with those obtained from p+p and Au+Au collision systems at the same center-of-mass energy. The higher-order cumulant ratios ($C_4/C_2$, $C_5/C_1$, and $C_6/C_2$) show an overall decreasing trend as a function of the charged-particle multiplicity across systems. The isobar results align with the Au+Au trends within uncertainties. The observations are compared with calculations from Lattice Gauge Theory (LGT) that include a quark-hadron crossover. The systematic behavior of the higher-order cumulant ratios shows that, overall, they progressively approach LGT predictions with increasing multiplicity within uncertainties. This could imply that the medium created in these heavy-ion collisions gradually evolves into thermalized QCD matter undergoing a crossover transition with multiplicity.