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 ($\Delta \gamma$) 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 $\Delta \gamma$ are observed. At large $m_{\mathrm{inv}}$ where this background is significantly reduced, the $\Delta \gamma$ 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 $\Delta {\gamma }_{\mathrm{sig}}$ = (0.03 $\pm$ 0.06 $\pm$ 0.08) $\times 10^{-4}$, or $(2\pm 4\pm 5)\%$ of the inclusive $\Delta \gamma (m_{\mathrm{inv}}>0.4$ 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 $-1<\eta <-0.05$ and $0.05<\eta <1$. 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.

arXiv commentPRC published version

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