Deciphering the RΨm correlator in search for the chiral magnetic effect in relativistic heavy ion collisions

Background: The chiral magnetic effect (CME) is extensively studied in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (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 root s(NN) = 200 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 with STAR data as well as predict the degree of sensitivity of R-Psi 2 to isobar collisions with the event statistics taken at RHIC. Results: Our AMPT results show that the R-Psi 2 in Au + Au collisions is concave and apparently different from R-Psi 3, in contradiction to the findings in STAR's preprint, while the R-Psi 2 in p + Au and d + Au collisions are slightly concave. Our toy model ESE analysis indicates that the R-Psi 2 is sensitive to the event-by-event anisotropy q(2) as well as the elliptic flow parameter v(2). The toy model results further show that R(Psi 2 )depends on both the CME signal and the flow backgrounds, similar to the Delta gamma observable. It is found that the R-Psi 2 and Delta gamma observables show similar sensitivities and centrality dependencies in isobar collisions. Conclusions: Our AMPT results contradict those from a recent preprint by STAR. Our toy model simulations demonstrate that R-Psi 2 is sensitive to both the CME signal and physics backgrounds. Toy model simulations of isobar collisions show similar centrality dependence and magnitudes for the relative R(Psi 2 )strengths as well as the relative Delta gamma strengths. We conclude that R(Psi 2 )and the inclusive Delta gamma are essentially the same.