We review the recent measurements of the rare pion decays: π + → π 0 e + ν (beta, π e3 , or π β decay), radiative decay π + → e + νγ (π e2γ or RPD), and π + → e + ν (π e2 ), as well as the radiative muon decay, μ → eν νγ, their theoretical implications, and prospects for further improvement.
Thanks to exceptionally well controlled theoretical uncertainties, decays of light mesons, particularly of the pion, are understood at very high precision, typically a few parts per 10 4 , or better (see, e.g., . Hence, pion decays present fertile ground for testing predictions of the standard model (SM), as well as for setting constraints on processes and particles outside the SM. Muon decays are theoretically cleaner yet, and provide direct information concerning the symmetry properties of the weak interaction itself, e.g., departures from its V -A form.
The PIBETA experiment, with measurements in 1999-2001 and 2004 at the Paul Scherrer Institute (PSI), Switzerland, was primarily designed to improve the accuracy of the π β decay branching ratio. Pion decays at rest were detected in an detector system [4,5] shown schematically in Fig. 1. Normalizing to the number of observed π + → e + ν (π e2 ) decays, we determined the branching ratio value B ex-n (π + → π 0 e + ν) = 1.036(4) stat (4) syst (3) e2 × 10 -8 , where the first uncertainty is statistical, the second systematic, and the third arises from ΔB(π e2 ), experimental π e2 branching ratio uncertainty [6]. Normalizing instead to the more precise theoretical value of B(π e2 ) [3] yields B th-n (π + → π 0 e + ν) = 1.040(4) stat (4) syst × 10 -8 . Both results agree well with the SM prediction, and represent the best test to date of vector current conservation (CVC) in a meson.
Concurrently with the π e3 decay, the PIBETA collaboration has measured the π + → e + νγ (RPD) branching ratio over a wide region of phase space. Two sets of amplitudes contribute to RPD: the inner-bremsstrahlung, IB, fully described by QED, and the structure-dependent amplitude, SD. The standard V-A electroweak theory requires only two pion form factors, F A , axial vector, and F V , vector, to describe the SD amplitude. The vector form factor is strongly constrained by the CVC hypothesis to F V = 0.0259(9).
Minimum-χ 2 fits to the measured (E e + , E γ ) energy distributions result in the weak form factor value of F A = 0.0119(1) with a fixed value of F V = 0.0259. An unconstrained * Session in honor of the 85th birthday of Ernest M. Henley.