Production of electron--positron pairs, muon pairs, and gamma ray pairs from electron--positron annihilation at center of mass energies near the psi(3095) rest energy

The cross sections for the production of electron-positron pairs, muon pairs and gamma ray pairs from electron-positron annihilation were measured at center of mass energies near the PSI (3095) rest energy. The electron-positron storage ring SPEAR was used to provide the colliding beams. The experimental apparatus consisted of two identical spectrometers oriented in a collinear configuration at right angles with the beam. Each spectrometer consisted of five multiwire proportional chambers, an optional gamma ray converter, two plastic scintillation counters, a large sodium iodide total absorption shower counter and a thick steel absorber. Enhancements in the cross sections for electron-positron and muon pair production were observed, but not in the cross section for pairs of gamma rays. It is concluded that the PSI(3095) decays with approximately equal probability into electron-positron pairs and muon pairs, but not into gamma ray pairs