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

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. ...

Comments: 18 pages, 13 figures