Pair invariant mass to isolate background in the search for the chiral magnetic effect in Au + Au collisions at sNN=200 GeV

Le résumé fourni par la source

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 $(\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma})$ 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 $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{{}_{\mathrm{NN}}}}=200$ GeV by the STAR experiment at the BNL Relativistic Heavy Ion Collider. Strong resonance background contributions to $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$ are observed. At large ${m}_{\mathrm{inv}}$ where this background is significantly reduced, the $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}$ 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 $\mathrm{\ensuremath{\Delta}}{\ensuremath{\gamma}}_{\mathrm{sig}}=(0.03\ifmmode\pm\else\textpm\fi{}0.06\ifmmode\pm\else\textpm\fi{}0.08)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, or $(2\ifmmode\pm\else\textpm\fi{}4\ifmmode\pm\else\textpm\fi{}5)%$ of the inclusive $\mathrm{\ensuremath{\Delta}}\ensuremath{\gamma}({m}_{\mathrm{inv}}>0.4$ $\mathrm{GeV}/{c}^{2})=(1.58\ifmmode\pm\else\textpm\fi{}0.02\ifmmode\pm\else\textpm\fi{}0.02)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, within pion ${p}_{T}=0.2$--0.8 $\mathrm{GeV}/c$ and averaged over pseudorapidity ranges of $\ensuremath{-}1<\ensuremath{\eta}<\ensuremath{-}0.05$ and $0.05<\ensuremath{\eta}<1$. This represents an upper limit of $0.23\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, or $15%$ of the inclusive result, at $95%$ confidence level for the ${m}_{\mathrm{inv}}$-integrated CME contribution.

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