Branching ratio for the rare pion decay into positron and neutrino

A new precision measurement of the branching ratio of the rare pion decay into a positron and a neutrino (π→eν) has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a 4π BGO calorimeter. 3×${10}^5$ rare decays and 1.2×${10}^6$ normal pion decays (π→μν) were recorded. The branching ratio was finally calculated from 1.2×${10}^5$ 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±0.005)×${10}^{\mathrm{-}4}$ are 0.28% statistical and 0.29% systematical.