Precision Tests of Fundamental Interactions

Improved measurement of the π→eν branching ratio

Improved measurement of the π→eν branching ratio: In Physical Review Letters (2015), the PIENU collaboration announced an interim result (based on 10% of the data set) which improved knowledge of the branching ratio, and consequently of the equivalence of electron and muon couplings to the weak force, by a factor of two. Since the result agreed well with the Standard Model (SM) prediction assuming universality, it serves to deepen the lepton universality puzzle. It also further constrains hypothetical non-SM theories by increasing the mass-scale limitation on those theories. The subject is highly topical since tentative measurements in B-meson decays are indicating possible deviations from universality involving the third flavor or generation of particles. This work is the latest in a long series of TRIUMF experiments on pion decay, improving the precision by more than an order of magnitude. The final results are expected in late 2018.