Search for the chiral magnetic effect through beam energy dependence of charge separation using event shape selection

TL;DR

This study investigates the chiral magnetic effect in heavy-ion collisions by measuring charge separation signals across different energies, employing a novel event shape selection to reduce background influences.

Contribution

It introduces an event shape selection method to better isolate CME signals and provides new measurements of charge separation at various collision energies.

Findings

Finite charge separation signals observed at 11.5, 14.6, and 19.6 GeV with high significance.

Background suppression reduces the charge separation signal to no more than 20% of the original.

Results at other energies are consistent with zero, indicating energy dependence of the effect.

Abstract

High-energy, heavy-ion collisions can create local domains of chirality-imbalanced quarks, reflecting the topological features of quantum chromodynamics. The chiral magnetic effect (CME) predicts an electric charge separation of quarks in such topological domains along the magnetic field ($\vec{B}$) generated by the passing of two high-$Z$ nuclei. We use a correlation observable $\Delta\gamma^{112}$ between charged meson pairs to detect the CME-induced charge separation and a novel event shape selection (ESS) method to mitigate the background effects related to elliptic flow ($v_2$). The ESS method classifies events based on the emission pattern of final-state particles and determines $\Delta\gamma^{112}_{\rm ESS}$ from the zero-flow limit. We reconstruct the $\vec{B}$ field direction from the spectator nucleons, which minimizes backgrounds unrelated to the collective motion of the system. In this work, we report the measurements of $\Delta\gamma^{112}$ and a background indicator $\Delta\gamma^{132}$ in Au+Au collisions from the RHIC Beam Energy Scan phase II and at the top RHIC energy. After background suppression, $\Delta\gamma^{132}_{\rm ESS}$ aligns with zero, and $\Delta\gamma^{112}_{\rm ESS}$ is reduced to no more than 20\% of $\Delta\gamma^{112}$. We observe a finite residual charge separation with $2.6\sigma$, $3.1\sigma$, and $3.3\sigma$ significance in the 20\%--50\% centrality range of Au+Au collisions at 11.5, 14.6, and 19.6 GeV. The results at 17.3 and 27 GeV also show positive values but with a lower significance of $1.3\sigma$ and $1.1\sigma$, respectively. The corresponding $\Delta\gamma^{112}_{\rm ESS}$ values at 7.7, 9.2, and 200 GeV are consistent with zero within uncertainties.