Measurement of Fifth- and Sixth-Order Fluctuations of (Net-)proton Number in Au+Au Collisions from Phase II of the Beam Energy Scan Program at RHIC

TL;DR

This paper presents high-statistics measurements of high-order fluctuations of (net-)proton numbers in heavy-ion collisions across a range of energies, providing insights into QCD phase structure and baryon number fluctuations.

Contribution

It reports the first detailed measurement of fifth- and sixth-order factorial cumulants of (net-)proton multiplicity distributions in Au+Au collisions at RHIC, testing theoretical predictions and constraining QCD phase transition models.

Findings

Cumulants increase with order but show no sign alternation.

Cumulant ratios fluctuate around zero, consistent with some theoretical models.

Results align with lattice QCD and HRG predictions at higher energies, with UrQMD matching lower energy data.

Abstract

We report high-statistics measurements of fifth- and sixth-order factorial cumulants and cumulant ratios of (net-)proton multiplicity distributions in Au+Au collisions at $\sqrt{s_{NN}} = 7.7$--27 GeV, using data from the STAR experiment collected during the Beam Energy Scan Phase~II at RHIC. Protons and antiprotons are identified at midrapidity ($|y| < 0.5$) with transverse momentum $0.4 < p_T < 2.0$ GeV/$c$. The proton factorial cumulants $\kappa_4$, $\kappa_5$, and $\kappa_6$ increase with order but exhibit no sign alternation within current uncertainties, offering no evidence for a two-component structure in the proton multiplicity distribution, as might be expected near a first-order phase transition. The cumulant ratios $C_{5}/C_{1}$ and $C_{6}/C_{2}$ fluctuate around zero in collisions at 0--40\% centrality. The results are consistent with both the negative predictions from lattice QCD (LQCD) and the positive trends obtained from the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. At $\sqrt{s_{NN}} \gtrsim 27$ GeV, the $C_4/C_2$ and $C_5/C_1$ results are compatible with predictions from lattice QCD, functional renormalization group (FRG), and hadron resonance gas (HRG) models, while UrQMD describes the data better at lower energies. These measurements place constraints on baryon number fluctuations and offer valuable insights into the QCD phase structure.