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

Department of Physics and Astronomy, Purdue University

^* e-mail: zhao656@purdue.edu

Published online: 26 January 2018

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 (Δ_ϒ) is the intrinsic particle correlations (such as resonance decays) coupled with the azimuthal elliptical anisotropy (v₂). In heavy-ion collisions, the magnetic field direction and event plane angle are correlated, thus the CME and the v₂-induced background are entangled. In this report, we present two studies from STAR to shed further lights on the background issue. (1) The Δ_ϒ 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 Δ_ϒ 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 v₂ scaled correlator is similar. (2) A new approach is devised to study Δ_ϒ 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.