Unexplained Excess of Electron-Like Events From a 1-GeV Neutrino Beam

The MiniBooNE Collaboration observes unexplained electron-like events in the reconstructed neutrino energy range from 200 to 475 MeV. With $6.46 \times 10^{20}$ protons on target, 544 electron-like events are observed in this energy range, compared to an expectation of $415.2\pm 43.4$ events, corresponding to an excess of $128.8\pm 20.4\pm 38.3$ events. The shape of the excess in several kinematic variables is consistent with being due to either ${\nu }_e$ and ${\bar{\nu }}_e$ charged-current scattering or to ${\nu }_{\mu }$ neutral-current scattering with a photon in the final state. No significant excess of events is observed in the reconstructed neutrino energy range from 475 to 1250 MeV, where 408 events are observed compared to an expectation of $385.9\pm 35.7$ events.