Background evaluations for the chiral magnetic effect with normalized correlators using a multiphase transport model

Abstract

The chiral magnetic effect (CME) induces an electric charge separation in a chiral medium along the magnetic field that is mostly produced by spectator protons in heavy-ion collisions. The experimental searches for the CME, based on the charge-dependent angular correlations ($γ$), however, have remained inconclusive, because the non-CME background contributions are not well understood. Experimentally, the $γ$ correlators have been measured with respect to the second-order ($Ψ_{2}$) and the third-order ($Ψ_{3}$) symmetry planes, defined as $γ_{112}$ and $γ_{123}$, respectively. The expectation was that with a proper normalization, $γ_{123}$ would provide a data-driven estimate for the background contributions in $γ_{112}$. In this work, we calculate different harmonics of the $γ$ correlators using a charge-conserving version of a multiphase transport (AMPT) model to examine the validity of the said assumption. We find that the pure-background AMPT simulations do not yield an equality in the normalized $γ_{112}$ and $γ_{123}$, quantified by $κ_{112}$ and $κ_{123}$, respectively. Furthermore, we test another correlator, $γ_{132}$, within AMPT, and discuss the relation between different $γ$ correlators.