The back-to-back relative excess observable in search for the chiral magnetic effect

Abstract

$\\textbf{Background:}$ \nThe chiral magnetic effect (CME) is extensively studied in heavy-ion collisions at RHIC and LHC. In the commonly used reaction plane (RP) dependent, charge dependent azimuthal correlator ($\\Delta\\gamma$), both the close and back-to-back pairs are included. Many backgrounds contribute to the close pairs (e.g. resonance decays, jet correlations), whereas the back-to-back pairs are relatively free of those backgrounds. \n$\\textbf{Purpose:}$ \nIn order to reduce those backgrounds, we propose a new observable which only focuses on the back-to-back pairs, namely, the relative back-to-back opposite-sign (OS) over same-sign (SS) pair excess ($r_{_\\text{BB}}$) as a function of the pair azimuthal orientation with respect to the RP ($\\varphi_{_\\text{BB}}$). \n$\\textbf{Methods:}$ \nWe use analytical calculations and toy model simulations to demonstrate the sensitivity of $r_{_\\text{BB}}(\\varphi_{_\\text{BB}})$ to the CME and its insensitivity to backgrounds. \n$\\textbf{Results:}$ \nWith finite CME, the $\\varphi_{_\\text{BB}}$ distribution of $r_{_\\text{BB}}$ shows a clear characteristic modulation. Its sensitivity to background is significantly reduced compared to the previous $\\Delta\\gamma$ observable. The simulation results are consistent with our analytical calculations. \n$\\textbf{Conclusions:}$ \nOur studies demonstrate that the $r_{_\\text{BB}}(\\varphi_{_\\text{BB}})$ observable is sensitive to the CME signal and rather insensitive to the resonance backgrounds.