Measurement of the branching ratio $\Gamma(\overline{p}p\to e^{+}e^{-})/\Gamma(\overline{p}p\to total)$ in antiproton-proton annihilation at rest

Analyzing a sample of 220000 events from an experiment still in progress at the CERN Proton Synchrotron, 60 pairs of electrons with an energy above 700 MeV have been observed. The electrons, produced by annihilation of antiproton stopped in a liquid-hydrogen target, are detected with optical spark chambers and scintillation counters. Twenty-nine out of these 60 pairs have been found to be collinear; normalizing with respect to the hadronic two-body channels pi /sup +/ pi /sup -/ and K/sup +/K/sup -/, a branching raio B/sub ee/= Gamma (pp to e/sup +/e/sup -/) Gamma /(pp to total)=(3.2+or-0.9)*10/sup -7/ has been obtained. (11 refs).