Precise measurement of the Pi+ -> Pi0 e+nu branching ratio

Using a large acceptance calorimeter and a stopped pion beam we have made a precise measurement of the rare Pi+ --> Pi0 e+ nu decay branching ratio. We have evaluated the branching ratio by normalizing the number of observed pion beta decays to the number of observed Pi+ --> e+ nu decays. We find the value of Gama(Pi+ --> Pi0 e+ nu)/Gama (total) = [1:036 +/- 0:004 (stat:) +/- 0:004 (syst:) +/- 0:003 ] x10-8, where the first uncertainty is statistical, the second systematic, and the third is the Pi+ -> e+ nu branching ratio uncertainty. Our result agrees well with the Standard Model prediction.