This thesis presents the first result of a search for massive neutrinos in π+→e+ν decay in the PIENU experiment at the 500 MeV proton cyclotron in TRIUMF (Canada’s National Laboratory for Particle and Nuclear Physics).
Recently our understanding of neutrino physics has been greatly improved with many experimental results. We know now that neutrinos have masses though they are really small. However, then some questions are worth asking. Why do they have such small masses compared to other particles? Why are there no right-handed neutrinos? Existence of massive neutrinos have a possibility to answer these questions. However, there are no evidence of massive neutrinos in any experiments up to now.
We analyzed the first physics data which were taken from May, 2009 to September, 2009 in the PIENU experiment. No evidence of massive neutrinos were observed and the upper limits of mixing parameter of massive neutrinos |Uei|2 were evaluated in the mass range 90MeV/c2-130MeV/c2. The upper limits (90% C.L.) were obtained to be |Uei|2<5x10-8 in the mass range 90MeV/c2- 110MeV/c2. This represents an improvement of a factor 1.5 over the current experimental limit.