Pair invariant mass to isolate background in the search for the chiral magnetic effect in Au+Au collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}$= 200 GeV

Abstract

Quark interactions with topological gluon configurations can induce local chirality imbalance and parity violation in quantum chromodynamics, which can lead to the chiral magnetic effect (CME) -- an electric charge separation along the strong magnetic field in relativistic heavy-ion collisions. The CME-sensitive azimuthal correlator observable ($\hat{I}”{\hat{I}}^3$) is contaminated by background arising, in part, from resonance decays coupled with elliptic anisotropy ($v_2$). We report here differential measurements of the correlator as a function of the pair invariant mass ($m_{\mathrm{inv}}$) in 20-50\% centrality Au+Au collisions at $\sqrt{s_{{}_{\mathrm{NN}}}}$= 200 GeV by the STAR experiment at RHIC. Strong resonance background contributions to $\hat{I}”{\hat{I}}^3$ are observed. At large $m_{\mathrm{inv}}$ where this background is significantly reduced, the $\hat{I}”{\hat{I}}^3$ value is found to be significantly smaller. An event-shape-engineering technique is deployed to determine the $v_2$ background shape as a function of $m_{\mathrm{inv}}$. We extract a $v_2$-independent and $m_{\mathrm{inv}}$-averaged signal $\hat{I}”{\hat{I}}_{\mathrm{sig}}^3$ = (0.03 $\pm$ 0.06 $\pm$ 0.08) $\times 10^{-4}$, or $(2\pm 4\pm 5)\%$ of the inclusive $\hat{I}”{\hat{I}}^3(m_{\mathrm{inv}}>0.4$ GeV/$c^2$)$=(1.58\pm 0.02\pm 0.02)\times 10^{-4}$, within pion $p_T$ = 0.2 - 0.8~\gevc and averaged over pseudorapidity ranges of $-1<\hat{I}\cdot <-0.05$ and $0.05<\hat{I}\cdot <1$. This represents an upper limit of $0.23\times 10^{-4}$, or $15\%$ of the inclusive result, at $95\%$ confidence level for the $m_{\mathrm{inv}}$-integrated CME contribution.

PRC published version