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 ($\Delta\gamma$) signals consistent with CME have been first observed almost a decade ago. It has also been known since then that the $\Delta\gamma$ is contaminated by a major background from resonance decays coupled with elliptic flow. In this contribution, we propose differential $\Delta\gamma$ measurements as function of the pair invariant mass ($m_{\rm inv}$). The $\Delta\gamma$ in the high $m_{\rm inv}$ region is essentially free of resonance backgrounds. In the low $m_{\rm inv}$ region, the $\Delta\gamma$ backgrounds show resonance peaks. The CME signal, presumably smooth in $m_{\rm inv}$, may thus be extracted from a two-component 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.