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تسجيل مستخدم جديدMeasurement of $eta^primerightarrow pi^+pi^- e^+e^-$ and $eta^primerightarrow pi^+pi^-mu^+mu^-$
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Based on a sample of 225.3 million J/psi events accumulated with the BESIII detector at the BEPCII, the decays of eta to pi+pi-l+l- are studied via J/psi to gammaeta. A clear eta signal is observed in the pi+pi-e+e- mass spectrum, and the branching fraction is measured to be BR(eta to pi+pi-e+e-) = (2.11pm0.12 (stat.)pm0.15 (syst.))times10^{-3}, which is in good agreement with theoretical predictions and the previous measurement, but is determined with much higher precision. No eta signal is found in the pi+ pi- mu+ mu- mass spectrum, and the upper limit is determined to be BR(eta to pi+ pi- mu+ mu-)<2.9times10^{-5} at the 90% confidence level.
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