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 $\simeq 25$ 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 \it in situ using control data samples prior to opening the `signal-box'. In the signal region, we observe \bf 155 events with a predicted background of \bf 129.2 $\pm$ 8.5 $\pm$ 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 $\pi^0$ decay, the measurement yields an upper limit on single photon events, \boldmath $< 4.0 \times 10^{-4}$ per \nm\ charged current event. Narrowing the search to events where the photon is approximately collinear with the incident neutrino, we observe \bf 78 events with a predicted background of \bf 76.6 $\pm$ 4.9 $\pm$ 1.9 yielding a more stringent upper limit, \boldmath $< 1.6 \times 10^{-4}$ per \nm\ charged current event.