Study of Rare Pion Decay

标准模型(SM)假定三代轻子的电弱耦合相等，这一假设被称为轻子普适性，在高精度测量介子衰变和其他反应中得到了高精度的研究。在SM中计算了介子衰变对电子和中微子的分支比(与普通衰变到包括辐射过程的µ子和中微子相比)，其极高的精度为rsm=1.252±0.0002×10-4。测量的分支比R对轻子普适性的破坏和在极高质量尺度O(1TeV)下的其他新物理过程高度敏感。最近，TRIUMF的PIENU实验在分析了10%的数据后，将R的实验知识提高了2%到0.2%。PIENU收集的大量数据集将使测量介子与电子和中微子分支比率的精度进一步提高两到四倍。这种衰变相对于普通衰变过程对一个Muon和中微子的分支比的实验精度水平预计为&lt；0.1%。努力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。我们之前在这个领域的实验已经给出了最严格的限制，即在介子衰变的质量范围内，与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.