Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2018-2019)

The standard model (SM) assumes equal electroweak couplings of the three lepton generations, a hypothesis known as lepton universality which is studied at high accuracy in high- precision measurements of pion decays and other reactions. The branching ratio of pion decays to electrons and neutrinos (compared to the ordinary decay to muons and neutrinos including radiative processes), has been calculated in the SM with extraordinary precision to be RSM=1.252±0.0002 x10-4. The measured branching ratio R is highly sensitive to violations of lepton universality and other new physics processes at very high mass scales O(1 TeV). The PIENU experiment at TRIUMF recently improved experimental knowledge of R by a factor of two to 0.2% based on analysis of 10% of the data taken. The large data set collected by PIENU will enable further improvement of the precision in the measurement of the branching ratio of the pions to electrons and neutrinos by another factor of two to four. The level of experimental precision on the branching ratio of this decay relative to the ordinary decay process to a muon and neutrino is expected to be <0.1%. Another focus of the efforts will be to significantly improve the search for heavy neutrinos coupled to electrons. Our previous experiments in this field already give the most stringent limits on the presence of heavy neutrinos coupled to electrons in the mass range accessible to pion decay.

The proposed grant will support the final analysis of the complete PIENU data set with the assistance of one postdoctoral research associate and one Ph. D. student. Analysis of data from the PIENU experiment will be completed during the proposed grant period.