Chiral magnetic effect search in p+Au, d+Au and Au+Au collisions at RHIC

TL;DR

This study investigates the presence of the chiral magnetic effect in various heavy-ion collision systems at RHIC, analyzing charge separation signals and background contributions to identify potential CME evidence.

Contribution

The paper introduces new analysis methods to distinguish CME signals from background effects in heavy-ion collisions, including studies in small systems and invariant mass dependence.

Findings

Significant charge separation observed in small systems suggests background dominance.

CME signal consistent with zero at high invariant mass.

Resonance backgrounds are identified as major contributors to charge separation signals.

Abstract

Metastable domains of fluctuating topological charges can change the chirality of quarks and induce local parity violation in quantum chromodynamics. This can lead to observable charge separation along the direction of the strong magnetic field produced by spectator protons in relativistic heavy-ion collisions, a phenomenon called the chiral magnetic effect (CME). A major background source for CME measurements using the charge-dependent azimuthal correlator ($\Delta\gamma$) is the intrinsic particle correlations (such as resonance decays) coupled with the azimuthal elliptical anisotropy ($v_{2}$). In heavy-ion collisions, the magnetic field direction and event plane angle are correlated, thus the CME and the $v_{2}$-induced background are entangled. In this report, we present two studies from STAR to shed further lights on the background issue. (1) The $\Delta\gamma$ should be all background in small system p+Au and d+Au collisions, because the event plane angles are dominated by geometry fluctuations uncorrelated to the magnetic field direction. However, significant $\Delta\gamma$ is observed, comparable to the peripheral Au+Au data, suggesting a background dominance in the latter, and likely also in the mid-central Au+Au collisions where the multiplicity and $v_{2}$ scaled correlator is similar. (2) A new approach is devised to study $\Delta\gamma$ as a function of the particle pair invariant mass ($m_{inv}$) to identify the resonance backgrounds and hence to extract the possible CME signal. Signal is consistent with zero within uncertainties at high $m_{inv}$. Signal at low $m_{inv}$, extracted from a two-component model assuming smooth mass dependence, is consistent with zero within uncertainties.