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 ($m_{\rm inv}$) in 20-50% centrality Au+Au collisions at $\sqrt{s_{_{\rm NN}}}$= 200 GeV by the STAR experiment at RHIC. Strong resonance background contributions to $\Delta\gamma$ are observed. At large $m_{\rm inv}$ 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 $m_{\rm inv}$. We extract a $v_2$-independent and $m_{\rm inv}$-averaged signal $\Delta\gamma_{\rm sig}$ = (0.03 $\pm$ 0.06 $\pm$ 0.08) $\times10^{-4}$, or $(2\pm4\pm5)\%$ of the inclusive $\Delta\gamma(m_{\rm inv}>0.4$ GeV/$c^2$)$ =(1.58 \pm 0.02 \pm 0.02) \times10^{-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 $0.23\times10^{-4}$, or $15\%$ of the inclusive result, at $95\%$ confidence level for the $m_{\rm inv}$-integrated CME contribution.

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10.17182/hepdata.95210.v1/t1

The $m_{\rm inv}$ dependences of the OS and SS pion pair multiplicities in 20-50$\%$ Au+Au collisions at 200 GeV.
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10.17182/hepdata.95210.v1/t2

The $m_{\rm inv}$ dependences of the $\gamma_{OS}$, $\gamma_{SS}$ in 20-50$\%$ Au+Au collisions at 200 GeV.
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10.17182/hepdata.95210.v1/t3

$m_{\rm inv}$ dependences of the relative excess of OS over SS pion pairs in 20-50$\%$ Au+Au collisions at 200 GeV.
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10.17182/hepdata.95210.v1/t4

$m_{\rm inv}$ dependences of $\Delta\gamma$ in 20-50$\%$ Au+Au collisions at 200 GeV.
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10.17182/hepdata.95210.v1/t5

The $\pi$ pair $\Delta\gamma$ at $m_{\rm inv} > m_{\rm inv}^{\rm low}$ for the data identified by TPC and more extended...
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10.17182/hepdata.95210.v1/t6

The elliptic flow $v_2$ in bins of $q_2$ for the three centrality bins in the 20-50$\%$ range of Au+Au collisions...
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10.17182/hepdata.95210.v1/t7

$m_{\rm inv}$ dependences of the $\Delta\gamma$ from ESE-selected event samples A (50$\%$ largest $q_{2}$) and B (50$\%$ smallest $q_{2}$) in...
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10.17182/hepdata.95210.v1/t8

$m_{\rm inv}$ dependences of the $\rm \Delta\gamma_{A} - \Delta\gamma_{B}$ in 20-50$\%$ Au+Au collisions at 200 GeV.
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10.17182/hepdata.95210.v1/t9

$\Delta\gamma_{\rm A}$ versus $\Delta\gamma_{\rm B}$ in 20-50$\%$ Au+Au collisions at 200 GeV. (The data are arranged as function of mass)

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