A new measurement of the branching ratio $R_{e/\mu }=\mathrm{\Gamma }({\pi }^{+}\to e^{+}\nu +{\pi }^{+}\to e^{+}\nu \gamma )/\mathrm{\Gamma }({\pi }^{+}\to {\mu }^{+}\nu +{\pi }^{+}\to {\mu }^{+}\nu \gamma )$ resulted in $R_{e/\mu }^{\text{exp}}=[1.2344\pm 0.0023(\text{stat})\pm 0.0019(\text{syst})]\times {10}^{-4}$. This is in agreement with the standard model prediction and improves the test of electron-muon universality to the level of 0.1%.

• Received 8 June 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.071801