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

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 $Δγ^{112}$ 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 $Δγ^{112}_{\rm ESS}$ 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 $Δγ^{112}$ and a background indicator $Δγ^{132}$ in Au+Au collisions from the RHIC Beam Energy Scan phase II and at the top RHIC energy. After background suppression, $Δγ^{132}_{\rm ESS}$ aligns with zero, and $Δγ^{112}_{\rm ESS}$ is reduced to no more than 20\% of $Δγ^{112}$. We observe a finite residual charge separation with $2.6σ$, $3.1σ$, and $3.3σ$ 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 $1.3σ$ and $1.1σ$, respectively. The corresponding $Δγ^{112}_{\rm ESS}$ values at 7.7, 9.2, and 200 GeV are consistent with zero within uncertainties.