Search

We have measured the π+→e+νγ branching ratio over a wide region of phase space, based on a total of 65 460 events acquired using the PIBETA detector. Minimum-χ2 fits to the measured (Ee+, Eγ) energy distributions result in the weak form factor value of FA=0.0119(1) with a fixed value of FV=0.0259. An unconstrained fit yields FV=0.0258(17) and FA=0.0117(17). In addition, we have measured a=0.10(6) for the dependence of FV on q2, the e+ν pair invariant mass squared, parametrized as FV(q2)=FV(0)(1+aq2). The branching ratio for the kinematic region Eγ>10 MeV and θe+γ>40° is measured to be Bexpt=73.86(54)×10-8. Earlier deviations we reported in the high-Eγ–low-Ee+ kinematic region are resolved without a tensor term. We also derive new values for the pion polarizability αE=2.78(10)×10-4 fm3 and neutral pion lifetime τπ0=(8.5±1.1)×10-17 s.

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 measurements 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., Comment: pdflatex, 11 pages, 5 figures, to be published in "Progress in High-Energy Physics and Nuclear Safety", NATO Science for Peace Series: B - Physics and Biophysics

We have used the PIBETA large-acceptance detector for a precise measurement of the pi+->e+nigama radiative decay at rest, with a broad phase space coverage. Using the CVC value for the pion vector form factor, F_V=0.0259(5), we have obtained a new value of the pion axial-vector form factor gamma=F_A/F_V=0.429(14). However, significant deviations from the SM predictions are evident in our data. The discrepancy can be accounted for by introducing a pion tensor form factor of F_T=-0.0016(3). The question of the true nature of the observed deviations remains open pending further theoretical and experimental study.

Using a large acceptance calorimeter and a stopped pion beam we have made a precise measurement of the rare Pi+ --> Pi0 e+ nu decay branching ratio. We have evaluated the branching ratio by normalizing the number of observed pion beta decays to the number of observed Pi+ --> e+ nu decays. We find the value of Gama(Pi+ --> Pi0 e+ nu)/Gama (total) = [1:036 +/- 0:004 (stat:) +/- 0:004 (syst:) +/- 0:003 ] x10-8, where the first uncertainty is statistical, the second systematic, and the third is the Pi+ -> e+ nu branching ratio uncertainty. Our result agrees well with the Standard Model prediction.

We have used the PIBETA detector at the PSI for a precise measurement of rare pion and muon weak decays. We have collected a large statistic al sample of (1) pi+->e+ni_e, (2) pi+->pi0e+ni_e, (3) pi+->e+ni_egama, (4) pi+->e+ni_eni_mi, and (5) pi+->e+ni_eni_migama decays. We have evaluated the aboslute branching ratiosfor these processes by normalizing to the independently measured number of decaying pi+'s (or pi+'s). We discuss the mutual consistency of the preliminary results.

We have studied radiative pion decays pi^+--e^+ni^gamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 41 601 events we have evaluated absolute pi e ni^gamma branching ratios in the three regions. Minimum Ksi^2 fits to the integral and differential (Ee+, Egamma) distributions result in the axial-to-vector weak form factor ratio of gamma FA/FV = 0.443(15), or FA = 0.0115(4) with FV = 0.0259. However, deviations from standard model predictions in the high-Egamma– low-Ee+ kinematic region indicate the need for further theoretical and experimental work.

The PIBETA detector at PSI is used to study the signals and backgrounds to rare pion and muon decays. We report the measured decay rates for \pi^+\to e^+\nu\gamma, \mu^+\to e^+\nu\nu\gamma, and \mu^+\to e^+\nu\nu e^+e^- processes, and discuss the effectiveness of the PIBETA detector in testing the limits for the lepton-family-number non-conserving muon decays. The extracted branching ratios are based on the statistics of 3.5\cdot 10^12 stopped \pi^+’ s collected in 1999 with pion beam intensities between 5\cdot 10^4\, \pi^+/s and 1\cdot 10^6\, \pi^+/s.