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

Abstract

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 (∆γ) 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 ∆γ 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 ∆γ 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 ∆γ 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.

Experimental searches for the chiral magnetic effect in heavy-ion collisions

“…The resulting jΔηj-averaged threeparticle correlators are shown in Part of the observed multiplicity (or centrality) dependence is understood as a dilution effect that falls with the inverse of event multiplicity [7]. The notably similar magnitude To eliminate sources of correlations that are charge independent (e.g., directed flow, v 1 ) and to explore a possible charge separation effect generated by the CME, the difference of three-particle correlators between the OS and SS is shown as a function of jΔηj in the multiplicity range Similar analysis has also been carried out at RHIC, using p+Au and d+Au collisions [176,177,178,179]. Figure 11 right panel shows the γ SS and γ OS correlators as functions of mid-rapidity charged hadron multiplicity density (dN ch /dη) in p+A and d+A collisions at √ s NN = 200 GeV, compared to Au+Au collisions at the same energy [136,137,138].…”

“…This can be achieved by differential measurements of the ∆γ as a function of the particle pair invariant mass (m inv ) to identify and remove the resonance decay backgrounds [165,204]. This method has not been explored until recently [177,178,205]. We discuss this method in Sect.…”

“…Removing resonance decay backgrounds by m inv cuts could enhance the sensitivity of the ∆γ measurements to potential CME signals. Figure 21 shows the preliminary results in mid-central Au+Au collisions by STAR [177,178,205]. The left panel shows the m inv dependence of the relative OS and SS pion pair abundance difference, r = (N OS − N SS )/N OS .…”

“…[165,204]. Right panel: from Run-11 STAR data as function of centrality [177,178,205]. Errors shown are statistical.…”

“…The m inv dependences of the CME signal and the background are distinctively different, and this can be exploited to extract CME signals at low m inv . The feasibility of this method was investigated by a toy-MC simulation [165] as well as in STAR data [177,178]. In principle, in order to extract the CME signal in the low-m inv region, the m inv dependence of the background contribution is needed.…”