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 () has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a 4 BGO calorimeter. 310^5 rare decays and 1. 210^6 normal pion decays () were recorded. The branching ratio was finally calculated from 1. 210^5 rare decays after various cuts in the time window from 7. 5 to 200 ns after pion stop. The errors of the result (1. 2350. 005) 10^-4 are 0. 28% statistical and 0. 29% systematical.