Measurement of High-Energy Direct Photons inψDecays

Abstract

The inclusive $\gamma$ and ${\pi }^0$ momentum distributions at the $\psi$ have been measured. Using these data and estimates of $\eta$ production, it is found that (4.1 ± 0.8)% of $\psi$ decays contain a direct photon with energy greater than 60% of the beam energy. The expected momentum distribution for direct photons calculated to lowest order in quantum chromodynamics is qualitatively different from that observed in the data.