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 ([pi][r arrow][ital e][nu]) has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a 4[pi] BGO calorimeter. 3[times]10[sup 5] rare decays and 1.2[times]10[sup 6] normal pion decays ([pi][r arrow][mu][nu]) were recorded. The branching ratio was finally calculated from 1.2[times]10[sup 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[plus minus]0.005)[times]10[sup [minus]4] are 0.28% statistical and 0.29% systematical.