Search for massive neutrinos in ${\pi }^{+}{\rightarrow }e^{+}{\nu }_{e}$ decay.

The PIENU experiment aims to measure the branching ratio of the charged pion decay with precision of < 0.1 %. This measurement is much sensitive to search for massive neutrinos coupled to electrons in ${\pi }^{+}{\rightarrow }e^{+}{\nu }_{e}$ decay. The initial analysis was completed and the upper limit on the neutrino mixing parameter | U | in the neutrino mass range of 0 to 55 MeV/ c was improved by a factor of 1.5, and the sensitivity for the mass range of 68 to 129 MeV/ c was improve by a factor of up to 4. [ABSTRACT FROM AUTHOR]