Precision Measurements with Pions—Review

The branching ratio of pion decays, R e / μ = Γ ( π + → e + ν + e + ν γ ) / Γ ( π + → μ + ν + μ + ν γ ) , has provided the best test of electron-muon universality in weak interactions. While the Standard Model prediction is R e / 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 R e / μ , as well as related precision measurements of other pion decays, are described.