Scaling properties of background- and chiral-magnetically-driven charge separation in heavy ion collisions at $\sqrt s_{\mathrm{NN}}=200$ GeV

TL;DR

This study uses the AVFD model and charge-sensitive correlators to analyze charge separation in heavy ion collisions, revealing scaling behaviors that help distinguish background effects from the chiral magnetic effect (CME).

Contribution

It demonstrates that the inverse variance of charge separation signals scales with charged-particle multiplicity, providing a new method to separate background from CME signals in collision data.

Findings

Background-related variance is event-shape independent and scales with inverse multiplicity.

CME-related signals show deviations from the expected scaling, indicating their presence.

Corrections to previous measurements suggest compatibility with CME contributions.

Abstract

The Anomalous Viscous Fluid Dynamics model, AVFD, is used in concert with the charge-sensitive correlator $R_{\Psi_2}(\Delta S)$ to investigate the scaling properties of background- and chiral-magnetically-driven (CME) charge separation ($\Delta S$), characterized by the inverse variance $\mathrm{\sigma^{-2}_{R_{\Psi_2}}}$ of the $R_{\Psi_{2}}(\Delta S)$ distributions obtained in collisions at $\sqrt s_{\mathrm{NN}}=200$ GeV. The $\mathrm{\sigma^{-2}_{R_{\Psi_2}}}$ values for the background are observed to be event-shape-independent. However, they scale with the reciprocal charged-particle multiplicity $(1/\left< N_{\rm ch} \right>)$, indicating an essential constraint for discerning background from the signal and a robust estimate of the difference between the backgrounds in Ru+Ru and Zr+Zr collisions. By contrast, the $\mathrm{\sigma^{-2}_{R_{\Psi_2}}}$values for signal + background show characteristic $1/\left< N_{\rm ch} \right>$ scaling violations that characterize the CME-driven contributions. Corrections to recent ${R_{\Psi_2}(\Delta S)}$ measurements \cite{STAR:2021mii} that account for the background difference in Ru+Ru and Zr+Zr collisions indicate a charge separation difference compatible with the CME. The results further suggest that $\mathrm{\sigma^{-2}_{R_{\Psi_2}}}$ measurements for peripheral and central collisions in concert with $1/\left< N_{\rm ch} \right>$ scaling, provides a robust constraint to quantify the background and aid characterization of the CME.