Measurement of the pion branching ratio at TRIUMF

The branching ratio of pion decays, R = Γ(π+→e+νe+π+→e+νeγ)Γ(π+→μ+νμ+π+→μ+νμγ is one of the most accurately calculated decay processes involving hadrons and has provided one of the most stringent tests of the hypothesis of electron-muon universality in weak interactions. The current experimental branching ratio Rexp = (1.230±0.004)×10−4 is 40 times less precise than the theoretical prediction RSM = 1.2352(1)10−4. The TRIUMF PIENU experiment aims at measuring the pion branching ratio with a precision improved by at least a factor five.