Measurement of the branching fraction of and search for a CP-violating asymmetry in η′→π+π−e+e− at BESIII

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Abstract

The rare decay ${\ensuremath{\eta}}^{\ensuremath{'}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{e}^{+}{e}^{\ensuremath{-}}$ is studied using a sample of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{9}\text{ }\text{ }J/\ensuremath{\psi}$ events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be $(2.42\ifmmode\pm\else\textpm\fi{}0.0{5}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}0.0{8}_{\mathrm{syst}})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. Due to the inclusion of new data, this result supersedes the last BESIII result on this branching fraction. In addition, the $CP$-violating asymmetry in the angle between the decay planes of the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$-pair and the ${e}^{+}{e}^{\ensuremath{-}}$-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is ${\mathcal{A}}_{CP}=(2.9\ifmmode\pm\else\textpm\fi{}3.{7}_{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}1.{1}_{\mathrm{syst}})%$, which is consistent with the standard model expectation of no $CP$-violation. The precision is comparable to the asymmetry measurement in the ${K}_{L}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{e}^{+}{e}^{\ensuremath{-}}$ decay where the observed $(14\ifmmode\pm\else\textpm\fi{}2)%$ effect is driven by a standard model mechanism.

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