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

The MiniBooNEexperiment at Fermilab reports results from an analysis of $\nu_e$ appearance data from $12.84 \times 10^{20}$ protons on target in neutrinomode, an increase of approximately a factor of two over previously reported results. A $\nu_e$ charged-currentquasielastic event excess of $381.2 \pm 85.2$ events ($4.5 \sigma$) is observed in the energy range $200 charged-currentquasielastic event excess of $460.5 \pm 99.0$ events ($4.7 \sigma$) is observed. If interpreted in a two- neutrino oscillationmodel, ${\nu}_{\mu} \rightarrow {\nu}_e$, the best oscillation fit to the excess has a probability of $21.1\%$, while the background-only fit has a $\chi^2$ probability of $6 \times 10^{-7}$ relative to the best fit. The MiniBooNEdata 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 MiniBooNEexcesses is $6.0 \sigma$. A two- neutrino oscillationinterpretation of the data would require at least four neutrinotypes and indicate physics beyond the three neutrinoparadigm.Although the data are fit with a two- neutrino oscillationmodel, other models may provide better fits to the data.