Simple dynamics, few available decay channels, and highly controlled radiative and loop corrections, make pion and muon decays a sensitive means of exploring details of the underlying symmetries. We review the current status of the rare decays:, and µ + → e + ννγ. For the latter we report new preliminary values for the branching ratio B(Eγ > 10 MeV, θeγ > 30 • ) = 4.365 (9)stat (42)syst × 10 −3 , and the decay parameterη = 0.006 (17)stat (18)syst, both in excellent agreement with standard model predictions. We review recent measurements, particularly by the PIBETA and PEN experiments, and near-term prospects for improvement. These and other similar precise low energy studies complement modern collider results materially.
Abstract. Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B πe2 ≡ R π e/µ , the π + → e + ν(γ) decay branching ratio, at the Paul Scherrer Institute, to reduce the present 40× experimental precision lag behind theory to ∼ 6 − 7×. Because of large helicity suppression, R π e/µ is uniquely sensitive to contributions from non-(V − A) physics, making this decay a particularly suitable subject of study. Even at current precision, the experimental value of B πe2 provides the most accurate test of lepton universality available. During runs in 2008-10, PEN has accumulated over 2 × 10 7 π e2 events; a comprehensive maximum-likelihood analysis is currently under way. The new data will also lead to improved precision of the earlier PIBETA results on radiative π and µ decays.
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