Charge Multiplicity Asymmetry Correlation Study Searching for Local Parity Violation at RHIC for STAR

TL;DR

This study investigates charge asymmetry correlations in heavy ion collisions at RHIC to search for local parity violation, finding that observed charge separation effects are likely influenced by event anisotropy rather than solely by the Chiral Magnetic Effect.

Contribution

The paper provides new experimental measurements of charge asymmetry correlations across different collision systems and energies, highlighting the influence of event anisotropy on charge separation signals.

Findings

Charge separation appears to be in-plane rather than out-of-plane.

Charge separation correlates with the event-by-event $v_2$ parameter.

Results are consistent with zero charge separation in events with $v_2 o 0$.

Abstract

It has been suggested that local parity violation in QCD would lead to charge separation of quarks by the Chiral Magnetic Effect (CME) in heavy ion collisions. Charge separation could yield a dynamical charge multiplicity asymmetry with respect to the reaction plane. In this talk, we report results on charge multiplicity asymmetry correlations in $\sqrt{s_{NN}}$ = 200 GeV Au+Au and d+Au collisions by the STAR experiment, as well as from the RHIC beam energy scan. We found that the correlation results could not be explained by CME alone. To gain further insights, we study our results as a function of the measured azimuthal angle range as well as the event-by-event anisotropy parameter $v_2$. The results indicate that the charge separation effect appears to be in-plane rather than out-of-plane. We found that the charge separation effect is proportional to the event-by-event $v_2$ and consistent with zero in events with $v_2 \approx 0$. Our studies suggest that the charge separation effect, within the statistical error, may be a net effect of event anisotropy and correlated particle production. Possible upper limit on the CME imposed by our data will be discussed.