Branching ratio for the rare pion decay into positron and neutrino

Abstract

A new precision measurement of the branching ratio of the rare pion decay into a positron and a neutrino (\ensuremath{\pi}\ensuremath{\rightarrow}e\ensuremath{\nu}) has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a 4\ensuremath{\pi} BGO calorimeter. 3\ifmmode\times\else\texttimes\fi{}${10}^{5}$ rare decays and 1.2\ifmmode\times\else\texttimes\fi{}${10}^{6}$ normal pion decays (\ensuremath{\pi}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu}) were recorded. The branching ratio was finally calculated from 1.2\ifmmode\times\else\texttimes\fi{}${10}^{5}$ rare decays after various cuts in the time window from 7.5 to 200 ns after pion stop. The errors of the result (1.235\ifmmode\pm\else\textpm\fi{}0.005)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ are 0.28% statistical and 0.29% systematical.