UrQMD Study of the Effects of Centrality Definitions on Higher Moments of Net Protons at RHIC

TL;DR

This study uses UrQMD simulations to analyze how different centrality definitions affect higher moments of net protons in heavy-ion collisions, revealing biases in multiplicity-based centrality measures.

Contribution

It demonstrates the impact of centrality definition choices on higher moment calculations, highlighting potential biases in experimental analyses.

Findings

UrQMD reproduces STAR's net proton moment measurements for certain energies and centralities.

Centrality based on multiplicity introduces biases in higher moment calculations.

Biases are minimal only in the most central collision bin (0-5%).

Abstract

A study using UrQMD is presented concerning the higher moments of net protons from Au+Au collisions at 7.7, 11.5, 14.6, 19.6, 27, 39, 62.4, and 200 GeV, concentrating on $C_{4}/C_{2} = \kappa\sigma^{2}$. Higher moments of net protons are predicted to be a sensitive probe of the critical point of QCD. At the QCD critical point, particular ratios of the moments of net protons are predicted to differ from the Poisson baseline. Recently STAR has published the higher moments of net protons for Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV. UrQMD quantitatively reproduces STAR's measured $C_{4}/C_{2} = \kappa\sigma^{2}$ for net protons for all Au+Au collisions more central than 30\% and at all centralities for $\sqrt{s_{\rm NN}}$ = 7.7 and 11.5 GeV. The effects are investigated of three different centrality definitions on the values of $C_{4}/C_{2} = \kappa\sigma^{2}$ from UrQMD calculations including the impact parameter given by UrQMD. It is shown that using a centrality definition based on multiplicity to calculate the higher moments of net protons gives a biased answer for $C_{4}/C_{2} = \kappa\sigma^{2}$, except in the most central bin (0-5\%).