Observation of direct photon production in a high energy neutrino experiment

In Fermilab experiment E-546, 8470 events of the type nu/sub μ/ N → μ-X were fully measured in the 15' bubble chamber. The bubble chamber was equipped with a two plane external muon identifier and was filled with a Ne-H2 mixture for good photon detection efficiency. The sample of primary photons whose momenta formed small angles in space with the muon momenta contained significantly more photons than the number expected from decays of neutral hadrons. The excess photons cannot be explained by bremsstrahlung of the muon as it passes through the liquid (outer bremsstrahlung) or by other conventional sources. The excess photons can be explained by direct photon production at the weak interaction vertex (inner bremsstrahlung). Results are given for the total number of excess photons and their distribution with respect to the photon energy and the angle between the photon and the muon. The author shows also distributions of the excess photons with respect to the neutrino energy, the muon momentum transferase to the neutrino, and the Bjorken scaling variables x and y. The authors discuss in detail several theories of direct photon production in high energy neutrino events. The authors find the data on the excess photons to be in good agreement with the predictions of these theories