First Results From the PIENU Experiment

The PIENU experiment at TRIUMF aims to measure the pion decay branching ratio, $R_{\pi}=\Gamma(\pi\rightarrow e\nu(\gamma))/\Gamma(\pi\rightarrow\mu\nu(\gamma))$ which provides a sensitive test of lepton universality and constraints on new physics scenarios. The theoretical uncertainty on the Standard Model prediction of $R_{\pi}^{SM}$ is 0.02\%, a factor of twenty smaller than the experimental uncertainty. First results from the PIENU experiment $R_{\pi}^{exp}=(1.2344\pm0.0023(stat)\pm 0.0019(syst))\times 10^{-4}$, are consistent with the SM prediction and represent a 0.1\% measurement of lepton non-universality. Analysis of additional data will allow increased precision up to 0.05\%.