Initial Results from the PIENU experiment

Abstract

The pion branching ratio, Rπ = γ(π+→e+ve+π+→e+veϒ) γ(π+→μ+, vμ+π+→μ+vμϒ) provides a sensitive test of lepton universality and constraints on many new physics scenarios. The theoretical uncertainty on the Standard Model prediction of Rπ is 0.02%, a factor of twenty smaller than the experimental uncertainty. The analysis of a subset of data taken by the PIENU experiment will be presented. The result, Rπ = (1.2344±0.0023(stat) ±0.0019(syst)) 10-4[1], is consistent with the Standard Model prediction and represents a 0.1% constraint on lepton non-universality.