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

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.

arXiv commentISMD 2017 conference proceeding contribution

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Cited by in corpus (7)

• Varying the chiral magnetic effect relative to flow in a single nucleus-nucleus collision
• Isolating the chiral magnetic effect from backgrounds by pair invariant mass
• Search for the Chiral Magnetic Effect in Relativistic Heavy-Ion Collisions
• Measurements of the chiral magnetic effect with background isolation in 200 GeV Au+Au collisions at STAR
• Responses of the chiral-magnetic-effect-sensitive sine observable to resonance backgrounds in heavy-ion collisions
• A novel invariant mass method to isolate resonance backgrounds from the chiral magnetic effect
• Back-to-back relative-excess observable in search for the chiral magnetic effect