Initial results from the PIENU experiment

The pion branching ratio, $R_\pi = \frac{ \Gamma(\pi^+ \rightarrow e^+ \nu_e + \pi^+\rightarrow e^+ \nu_e \gamma)}{\Gamma(\pi^+ \rightarrow \mu^+ \nu_{\mu} + \pi^+ \rightarrow \mu^+ \nu_{\mu} \gamma)}$, 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±0.0023(stat)±0.0019(syst)) · 10$^{−4}$, is consistent with the Standard Model prediction and represents a 0.1% constraint on lepton non-universality.