Probing the Standard Model via rare pion and muon decays

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 π+→π0e+ν decay branching ratio with the 0.55 % total uncertainty. The π+→e+νγ data set was used to extract weak axial and vector form factors FA and FV, yielding a significant improvement in the precision of FA and FV. The μ+→e+ννγ distributions were well described with the two-parameter (ρSM, ) 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.