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تسجيل مستخدم جديدMeasurement of the Branching Fraction of and Search for a CP-Violating Asymmetry in $eta to pi^+ pi^- e^+ e^-$ at BESIII
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The rare decay $eta to pi^+ pi^- e^+ e^-$ is studied using a sample of $1.3 times 10^9$ $J/psi$ events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be $left( 2.42pm0.05_{stat.}pm0.08_{syst.} right) times 10^{-3}$. 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 $pi^+pi^-$-pair and the $e^+e^-$-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is $mathcal{A}_{CP} = left(2.9pm3.7_{stat.}pm1.1_{syst.} right)%$, which is consistent with the standard model expectation of no CP-violation. The precision is comparable to the asymmetry measurement in the $K^0_Ltopi^+pi^-e^+e^-$ decay where the observed $(14pm2)%$ effect is driven by a standard model mechanism.
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