Measurement of the branching ratio for the rare decay. pi. /sup 0/. -->. e/sup +/e/sup -/

This paper reports the details of an experiment designed to detect the decay ..pi../sup 0/..-->..e/sup +/e/sup -/ and to measure its branching ratio. The experiment used a beam of 300-MeV/c ..pi../sup -/ mesons incident on a liquid-hydrogen target to produce neutral pions in the reaction ..pi../sup -/p..--> pi../sup 0/n. Electron-positron pairs were detected in a magnetic spectrometer by multiwire proportional chambers. A gas Cherenkov counter provided electron identification. The effective-mass distribution of e/sup +/e/sup -/ pairs was decomposed with the aid of a Monte Carlo simulation of the ..pi../sup 0/..-->..e/sup +/e/sup -/ signal and background processes. The result is 59 +- 21 events, which corresponds to GAMMA(..pi../sup 0/..-->..e/sup +/e/sup -/)/GAMMA(..pi../sup 0/..--> gamma gamma..) = (17 +- 6 +- 3) x 10/sup -8/, where the first error is due to statistics and the second is an estimate of systematic effects. The central value is almost four times the unitarity lower limit while existing calculations generally predict values no larger than twice the unitarity limit.