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

Jie Zhao

doi:10.1051/epjconf/201817201005

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 (Δϒ) 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 Δϒ 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 Δϒ 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 Δϒ as a function of the particle pair invariant mass (minv) to identify the resonance backgrounds and hence to extract the possible CME signal. Signal is consistent with zero within uncertainties at high minv. Signal at low minv, extracted from a two-component model assuming smooth mass dependence, is consistent with zero within uncertainties.

Highlights

Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics (QCD) [1,2,3]
The systematic uncertainties on γ are estimated by varying the track quality cuts, the correction method used for the detector non-uniform azimuthal acceptance, and the pT range of the reference particle c
Since the particle pseudorapidity density, dN/dη, is asymmetric in small systems, there are relatively more small ∆η pairs than in Au+Au collisions, larger correlations. We have checked this effect by weighting particles in computing the correlator such that the weighted dN/dη is symmetric, and found it insufficient to account for the observed difference

Summary

Introduction

Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics (QCD) [1,2,3]. In relativistic heavy-ion collisions, this can lead to observable electric charge separation along the direction of the strong magnetic field produced by spectator protons [4,5,6]. Background sources, on the other hand, contribute to small-system collisions as to heavy-ion collisions when measured with respect to the event plane reconstructed from mid-rapidity particle momenta (as a proxy to the ψPP). We present results from similar control experiments using p+Au and d+Au collisions at RHIC These results are analyzed as a function of the final-state particle multiplicity to shed light on the background to the CME measurements in heavy-ion collisions at RHIC. A two-component model fit to the low mass region is reported along with the fitted possible CME signals

Experiment setup and data analysis

Results from small systems

Results from the invariant mass method

Summary