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}{→}e{ν}) has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a 4{π} BGO calorimeter. 3×{}10⁵ rare decays and 1.2×{}10⁶ normal pion decays ({π}{→}{μ}{ν}) were recorded. The branching ratio was finally calculated from 1.2×{}10⁵ 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^-4 are 0.28% statistical and 0.29% systematical.