Precision Measurements with Pions—Review

The branching ratio of pion decays, Re/μ=Γ(π+→e+ν+e+νγ)/Γ(π+→μ+ν+μ+νγ), has provided the best test of electron-muon universality in weak interactions. While the Standard Model prediction is Re/μ=(1.2353±0.0001)×10−4, the existing experimental results, (1.2265±0.0056)×10−4 (TRIUMF) and (1.2346±0.0050)×10−4 (PSI), have still orders of magnitude worse uncertainties. Since the decay π+→e+ν is helicity-suppressed, it is extremely sensitive to a presence of pseudo-scalar couplings arising from a wide range of new physics up to a mass scale of 1000 TeV/c2. Two new experiments aiming at a precision level of for Re/μ, as well as related precision measurements of other pion decays, are described.