Estimate of a nonflow baseline for the chiral magnetic effect in isobar collisions at RHIC

TL;DR

This paper estimates a nonflow baseline for the chiral magnetic effect in isobar collisions at RHIC, helping to distinguish genuine CME signals from background effects.

Contribution

The study decomposes nonflow contributions to the CME-sensitive observable ratio using experimental data and simulations, providing a refined baseline estimate.

Findings

Nonflow effects can significantly influence the CME observable ratio.

A quantitative baseline for nonflow contributions was established.

Implications for interpreting CME signals in isobar collisions are discussed.

Abstract

Recently, STAR reported the isobar (${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$, ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) results for the chiral magnetic effect (CME) search. The Ru+Ru to Zr+Zr ratio of the CME-sensitive observable $\Delta\gamma$, normalized by elliptic anisotropy ($v_{2}$), is observed to be close to the inverse multiplicity ($N$) ratio. In other words, the ratio of the $N\Delta\gamma/v_{2}$ observable is close to the naive background baseline of unity. However, nonflow correlations are expected to cause the baseline to deviate from unity. To further understand the isobar results, we decompose the nonflow contributions to $N\Delta\gamma / v_{2}$ (isobar ratio) into three terms and quantify each term by using the nonflow in $v_{2}$ measurement, published STAR data, and HIJING simulation. From these studies, we estimate a nonflow baseline of the isobar ratio of $N \Delta\gamma / v_{2}$ for the CME and discuss its implications.