New studies of allowed pion and muon decays

Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B_πe2≡Re_/μ^π, 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, Re_/μ^π 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 × 10⁷ 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.