Probing the Standard Model via rare pion and muon decays

Abstract

The PIBETA collaboration has used a non-magnetic pure CsI calorimeter operating at the Paul Scherrer Institute to collect the world's largest sample of rare pion and muon decays. We have extracted the absolute pi+ -> pi0 e+ nu decay branching ratio with the 0.55 % total uncertainty. The pi+ -> e + nu gamma data set was used to extract weak axial and vector form factors F_A and F_V, yielding a significant improvement in the precision of F_A and F_V. The mu+ -> e+ nu nu gamma distributions were well described with the two-parameter (rho_SM,eta_bar=0) solution. These results bring major improvements in accuracy over the current Particle Data Group listings and agree well with the predictions of the Standard Model.