A new precision measurement of the branching ratio of the rare pion decay into a positron and a neutrino (tr ev) has been completed. A beam of positive pions was stopped in an active target of plastic scintillator surrounded by a 4z BOO calorimeter. 3X10 rare decays and 1.2x10 normal pion decays (tr pv) were recorded. The branching ratio was finally calculated from 1.2X10 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)

more »

... x 10 are 0.28% statistical and 0.29% systematical. PACS numbers: 13.20.Cz An experiment was performed to measure the branching ratio R for the rare decay tr ev(y) (i.e. , tr ev including tr evy), normalized to the normal pion decay tr pv(y) with high precision, testing the pe universality of the weak interaction. This rare decay has been treated theoretically many times and values for R have been published by several authors during the development of the theory of the weak interaction [1-3] and also later in the framework of gauge theories. The most recent predictions for R are as follows: Marciano and Sirlin [4], (1.233~0.004) X 10 Goldman and Wilson [5], (1.239~0.001) X10 Marciano [6], (1.2345+ 0.0010) X 10