Initial results from the PIENU experiment

The pion branching ratio, $R_{\pi} = \frac{\Gamma ( \pi^+ \to e^+ \nu_e + \pi^+ \to e^+ \nu_e \gamma)}{\Gamma (\pi^+ \to \mu^+ \nu_{\mu} + \pi^+ \to \mu^+ \nu_{\mu} \gamma)}$ , provides a sensitive test of lepton universality and constraints on many new physics scenarios. The theoretical uncertainty on the Standard Model prediction of $R_{\pi}$ 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, $R_{\pi} =( 1.2344 \pm 0.0023(\text{stat}) \pm 0.0019(\text{syst})) \cdot 10^{−4}$, is consistent with the Standard Model prediction and represents a 0.1% constraint on lepton non-universality.