A Search for Single Photon Events in Neutrino Interactions

Abstract We present a search for neutrino induced events containing a single, exclusive photon using data from the NOMAD experiment at the CERN SPS where the average energy of the neutrino flux is ≃ 25 GeV . The search is motivated by an excess of electron-like events in the 200–475 MeV energy region as reported by the MiniBooNE experiment. In NOMAD, photons are identified via their conversion to e + e − in an active target embedded in a magnetic field. The background to the single photon signal is dominated by the asymmetric decay of neutral pions produced either in a coherent neutrino–nucleus interaction, or in a neutrino–nucleon neutral current deep inelastic scattering, or in an interaction occurring outside the fiducial volume. All three backgrounds are determined in situ using control data samples prior to opening the ‘signal-box’. In the signal region, we observe 155 events with a predicted background of 129.2 ± 8.5 ± 3.3 . We interpret this as null evidence for excess of single photon events, and set a limit. Assuming that the hypothetical single photon has a momentum distribution similar to that of a photon from the coherent π 0 decay, the measurement yields an upper limit on single photon events, 4.0 × 10 − 4 per ν μ charged current event. Narrowing the search to events where the photon is approximately collinear with the incident neutrino, we observe 78 events with a predicted background of 76.6 ± 4.9 ± 1.9 yielding a more stringent upper limit, 1.6 × 10 − 4 per ν μ charged current event.