The pion branching ratio (R^π = [formula omitted] ) is an auspicious observable for a test of the standard model of particle physics (SM). Rπ has been calculated within this framework with high precision because the strong interaction dynamics cancel out in the ratio and the structure dependence only appears through electroweak corrections. Since the discovery of the electronic pion decay in 1958, Rπ was measured with increasing precision and confirmed the SM value of RπSM = 1.2352(2) x 10⁻⁴. However, the current experimental precision is 20 times worse than the theoretical one leaving a large window for potential new physics at “high-mass" scales (up to ∽1000 TeV).
The PIENU experiment aims at measuring Rπ with an improved precision by a factor larger than 5 over the previous experiment at TRIUMF (Rπexp = (1.2265 ± 0:0056) x 10⁻⁴) in order to confront the theoretical prediction at the 0.1% level. The result presented in this thesis focuses on a fraction of the data taken since the beginning of physics data taking in 2009. A blind analysis has been implemented in order to avoid a human bias. With this set of data, the procedure is established for the final analysis. An improvement by a factor 1.17, dominated by statistical uncertainty, has been reached in the branching ratio precision. If added to the current Particle Data Group value, the result of this analysis reduces the uncertainty on the branching ratio by ∽25%.Science, Faculty ofPhysics and Astronomy, Department ofGraduat
