Charge Separation Measurements in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7--200 GeV in Search of the Chiral Magnetic Effect

Abstract

The chiral magnetic effect in heavy-ion collisions predicts a charge separation signal along a magnetic field, which indicates local $P$ and $CP$ violations in the quark-gluon plasma. We report measurements of electric charge separation signals perpendicular to the spectator event plane in Au+Au collisions using high-statistics data from RHIC Beam Energy Scan II and top-energy runs. A novel event shape selection method is employed to suppress the flow-induced background. The residual charge separation signals near the zero-flow limit are positive in Au+Au collisions within the 20\%--50\% centrality range, with significance levels of $2.6\sigma$, $3.1\sigma$, and $3.3\sigma$ at $\sqrt{s_{NN}} =$ 11.5, 14.6, and 19.6 GeV, respectively. At other beam energies, the signals are either statistically limited or consistent with zero.