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

TL;DR

This study uses a multiphase transport model to evaluate background contributions to charge-dependent correlations in heavy-ion collisions, questioning the assumption that normalized third-order correlators can estimate backgrounds in CME searches.

Contribution

It provides a detailed analysis of charge-dependent correlators using AMPT, challenging previous assumptions about their normalization and background estimation in CME experiments.

Findings

Normalized $oldsymbol{ ext{γ}}_{112}$ and $oldsymbol{ ext{γ}}_{123}$ are not equal in AMPT simulations.

The AMPT model does not support the assumption that $oldsymbol{ ext{γ}}_{123}$ can serve as a background estimate for $oldsymbol{ ext{γ}}_{112}$.

Relations between different $oldsymbol{ ext{γ}}$ correlators are discussed within the 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 ($\gamma$), however, have remained inconclusive, because the non-CME background contributions are not well understood. Experimentally, the $\gamma$ correlators have been measured with respect to the second-order ($\Psi_{2}$) and the third-order ($\Psi_{3}$) symmetry planes, defined as $\gamma_{112}$ and $\gamma_{123}$, respectively. The expectation was that with a proper normalization, $\gamma_{123}$ would provide a data-driven estimate for the background contributions in $\gamma_{112}$. In this work, we calculate different harmonics of the $\gamma$ 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 $\gamma_{112}$ and $\gamma_{123}$, quantified by $\kappa_{112}$ and $\kappa_{123}$, respectively. Furthermore, we test another correlator, $\gamma_{132}$, within AMPT, and discuss the relation between different $\gamma$ correlators.