A novel invariant mass method to isolate resonance backgrounds from the chiral magnetic effect

Abstract

The Chiral Magnetic Effect (CME) refers to charge separation along a strong magnetic field, due to topological charge fluctuations in QCD. Charge correlation (∆γ) signals consistent with CME have been first observed almost a decade ago. It has also been known since then that the ∆γ is contaminated by a major background from resonance decays coupled with elliptic flow. In this contribution, we propose differential ∆γ measurements as function of the pair invariant mass (m inv ). The ∆γ in the high m inv region is essentially free of resonance backgrounds. In the low m inv region, the ∆γ backgrounds show resonance peaks. The CME signal, presumably smooth in m inv , may thus be extracted from a twocomponent model fit. We demonstrate the feasibility and effectiveness of this novel method by using the AMPT and toy-model Monte-Carlo simulations. We also discuss an application of the method in data analysis.

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

“…The easiest way to remove resonance contributions from ∆γ is, therefore, to restrict the measurements to the large-m inv region. AMPT model simulation shows that such a m inv cut, although significantly reducing the pair statistics, can eliminate essentially all resonance decay backgrounds [165,204]. This is shown in the left panel of Fig.…”

“…(2) The second innovative method is to make measurements where resonance contributions are small or can be identified and removed [165,204]. 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].…”

“…The m inv structures are similar in r and ∆γ; the ∆γ correlator traces the distribution of the resonances. [165,204]. Right panel: from Run-11 STAR data as function of centrality [177,178,205].…”

“…In order to do so, resonance contributions must be excluded. In a two-component model, the m inv dependence of the ∆γ can be expressed [165,204] as…”

“…Note that in this fit model, unlike the ESE method described in Sect. 5.1, the background is not required to be strictly proportional to v 2 [204]. As long as the backgrounds are different for different q 2 event classes, one can extract the background shape as function of m inv .…”