Significant Excess of Electronlike Events in the MiniBooNE Short-Baseline Neutrino Experiment

The MiniBooNE experiment at Fermilab reports results from an analysis of nu(e) appearance data from 12.84 x 10(20) protons on target in neutrino mode, an increase of approximately a factor of 2 over previously reported results. A nu(e) charged-current quasielastic event excess of 381.2 +/- 85.2 events (4.5 sigma) is observed in the energy range 200 < E nu(QE) < 1250 MeV. Combining these data with the (nu) over bar (e) appearance data from 11.27 x 10(20) protons on target in antineutrino mode, a total nu(e) plus (nu) over bar (e) charged-current quasielastic event excess of 460.5 +/- 99.0 events (4.7 sigma) is observed. If interpreted in a two-neutrino oscillation model nu(mu) -> nu(e), the best oscillation fit to the excess has a probability of 21.1%, while the background-only fit has a chi(.)2 probability of 6 x 10(-7) relative to the best fit. The MiniBooNE data are consistent in energy and magnitude with the excess of events reported by the Liquid Scintillator Neutrino Detector (LSND), and the significance of the combined LSND and MiniBooNE excesses is 6.0 sigma. A two-neutrino oscillation interpretation of the data would require at least four neutrino types and indicate physics beyond the three neutrino paradigm. Although the data are fit with a two-neutrino oscillation model, other models may provide better fits to the data.