Decipher the $R_{\Psi_{m}}$ correlator in search for the chiral magnetic effect in relativistic heavy ion collisions

TL;DR

This study critically examines the $R_{ ext{\Psi}_m}$ correlator's effectiveness in detecting the chiral magnetic effect in heavy-ion collisions, comparing it with the $ riangle\gamma$ method and testing its robustness against background effects using simulations.

Contribution

The paper provides a detailed analysis of the $R_{\Psi_m}$ observable's sensitivity to CME signals and backgrounds, challenging previous claims of its robustness and offering comparative insights with the $\triangle\gamma$ correlator.

Findings

$R_{\Psi_m}$ shows sensitivity to flow backgrounds in simulations.

The observable's ability to distinguish CME signals from backgrounds is limited.

Comparison with $\triangle\gamma$ reveals different sensitivities to background effects.

Abstract

Background: The chiral magnetic effect (CME) is extensively studied in heavy-ion collisions at RHIC and the LHC. An azimuthal correlator called $R_{\Psi_{m}}$ was proposed to measure the CME. By observing the same $R_{\Psi_{2}}$ and $R_{\Psi_{3}}$ (convex) distributions from A Multi-Phase Transport (AMPT) model, by contrasting data and model as well as large and small systems and by event shape engineering (ESE), a recent preprint (arXiv:2006.04251v1) from STAR suggests that the $R_{\Psi_{m}}$ observable is sensitive to the CME signal and relatively insensitive to backgrounds, and their Au+Au data are inconsistent with known background contributions. Purpose: We examine those claims by studying the robustness of the $R_{\Psi_{m}}$ observable using AMPT as well as toy model simulations. We compare $R_{\Psi_{m}}$ to the more widely used $\Delta\gamma$ azimuthal correlator to identify their commonalities and differences. Methods: We use AMPT to simulate Au+Au, p+Au, and d+Au collisions at $\sqrt{s_{NN}} = 200 \text{ GeV}$, and study the responses of $R_{\Psi_{m}}$ to anisotropic flow backgrounds in the model. We also use a toy model to simulate resonance flow background and input CME signal to investigate their effects in $R_{\Psi_{2}}$. Additionally we use the toy model to perform an ESE analysis to compare to STAR data as well as predict the degree of sensitivity of $R_{\Psi_{2}}$ to isobar collisions with the event statistics taken at RHIC. ...