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

TL;DR

This study measures charge separation in Au+Au collisions across various energies to search for the chiral magnetic effect, employing a novel background suppression method, and finds significant signals at specific energies.

Contribution

It introduces a new event shape selection technique to reduce background in charge separation measurements in heavy-ion collisions.

Findings

Positive charge separation signals observed at 11.5, 14.6, and 19.6 GeV with significance above 2.6σ.

Signals at other energies are statistically limited or consistent with zero.

Results support the presence of the chiral magnetic effect at certain collision energies.

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.