Study of Rare Pion Decay

标准模型（SM）假设三代轻子的电弱耦合相等，这一假设被称为轻子普适性，在高精度测量π介子衰变和其他反应时被高精度研究。π介子衰变到电子和中微子的分支比（与包括辐射过程的介子和中微子的普通衰变相比）已经在SM中以非常精确的方式计算为RSM=1.252±0.0002 x10-4。测量的分支比R是非常敏感的轻子普适性和其他新的物理过程在非常高的质量尺度O（1 TeV）的违规。最近，TRIUMF的PIENU实验基于对10%数据的分析，将R的实验知识提高了2倍至0.2%。PIENU收集的大型数据集将使测量π介子与电子和中微子的分支比的精度进一步提高2到4倍。这种衰变相对于普通衰变到μ子和中微子的分支比的实验精确度预计<0.1%。这项工作的另一个重点将是显著改善对与电子耦合的重中微子的搜索。我们以前在这一领域的实验已经给出了最严格的限制，在质量范围内的介子衰变的电子耦合的重中微子的存在。

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.