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

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 ${\sigma }_{{\mathrm{R}}_{{\mathrm{\Psi }}_2}}^{-2}$ of the $R_{{\Psi }_2}(\Delta S)$ distributions obtained in collisions at ${\sqrt{s}}_{\mathrm{NN}}=200$ GeV. The ${\sigma }_{{\mathrm{R}}_{{\mathrm{\Psi }}_2}}^{-2}$ values for the background are observed to be event-shape-independent. However, they scale with the reciprocal charged-particle multiplicity $(1/⟨N_{\mathrm{ch}}⟩)$, 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 ${\sigma }_{{\mathrm{R}}_{{\mathrm{\Psi }}_2}}^{-2}$values for signal + background show characteristic $1/⟨N_{\mathrm{ch}}⟩$ 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 ${\sigma }_{{\mathrm{R}}_{{\mathrm{\Psi }}_2}}^{-2}$ measurements for peripheral and central collisions in concert with $1/⟨N_{\mathrm{ch}}⟩$ scaling, provides a robust constraint to quantify the background and aid characterization of the CME.

6 pages, 5 figures, submitted for publication