Measurement of the tau lepton electronic branching fraction

The tau lepton electron branching fraction has been measured with the CLEO II detector at the Cornell Electron Storage Ring as ${\mathit{B}}_{\mathit{e}}$=0.1749±0.0014±0.0022, with the first error statistical and the second systematic. The measurement involves counting electron-positron annihilation events in which both taus decay to electrons, and normalizing to the number of tau-pair decays expected from the measured luminosity. Detected photons in these events constitute a definitive observation of tau decay radiation.