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 <0.1% for Re/μs, as well as related precision measurements of other pion decays, are described. © 2010.
Numao, T. (2011). Precision Measurements with Pions-Review. Nuclear Physics B - Proceedings Supplements, 210–211, 249–254. https://doi.org/10.1016/j.nuclphysbps.2010.12.085