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تسجيل مستخدم جديدMeasurement of the eta->pi+pi-e+e- decay branching ratio
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The reaction pd->3He eta at threshold was used to provide a clean source of eta mesons for decay studies with the WASA detector at CELSIUS. The branching ratio of the decay eta->pi+pi-e+e- is measured to be (4.3+/-1.3+/-0.4)x10^-4.
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The PEN Collaboration is conducting a new measurement of the pi+ -> e+ nu branching ratio at the Paul Scherrer Institute, with the goal uncertainty of delta B/B(pie2)=5E-4 or lower. At present, the combined accuracy of all published pie2 decay measur
ements lags behind the theoretical calculation by a factor of 40. In this contribution we report on the PEN detector configuration and its performance during two development runs done in 2007 and 2008.
We have studied the eta->pi+pi-e+e-(gamma) decay using about 1.7 fb^-1 collected by the KLOE experiment at the DAFNE phi-factory. This corresponds to about 72 millions eta mesons produced in phi radiative decays. We have measured the branching ratio,
inclusive of radiative effects, with 4% accuracy: BR(eta->pi+pi-e+e-(gamma)) = (26.8 +/- 0.9_Stat. +/- 0.7_Syst.) x 10^-5. We have obtained the first measurement of the CP-odd pipi-ee decay planes angular asymmetry, A_phi = (-0.6 +/- 2.5_Stat. +/- 1.8_Syst.) x 10^-2.
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 f
raction 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.
The branching ratio of the rare decay pi^0 -> e^+e^- has been measured in E799-II, a rare kaon decay experiment using the KTeV detector at Fermilab. The pi^0s were produced in fully-reconstructed K_L -> 3 pi^0 decays in flight. We observed 275 candid
ate pi^0 -> e^+e^- events, with an expected background of 21.4 +- 6.2 events which includes the contribution from Dalitz decays. We measured BR(pi^0 -> e^+e^-, x>0.95) = (6.09 +- 0.40 +- 0.24) times 10^{-8}, where the first error is statistical and the second systematic. This result is the first significant observation of the excess rate for this decay above the unitarity lower bound.
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m 0.0019 (syst)) times 10^{-4}$. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1 %.
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