Status of Pion Decay Experiments

Abstract

The branching ratio of pion decays, , has provided a sensitive test of electron-muon universality in weak interactions. The uncertainty of the Standard Model prediction is at a 0.01% level. Although a recent measurement, Re/μ = (1.2344 ± 0.0023(stat) ± 0.0019(syst)) × 10-4, reduced the experimental uncertainty by a factor of two, there is room for improvement by more than an order of magnitude. The status of two experiments at TRIUMF and PSI as well as related pion decay experiments is presented.