Event Excess in the MiniBooNE Search for ${\bar{\nu }}_{\mu }\to {\bar{\nu }}_e$ Oscillations

The MiniBooNE experiment at Fermilab reports results from a search for $\bar \nu_\mu \rightarrow \bar \nu_e$ oscillations, using a data sample corresponding to $5.66\times {10}^{20}$ protons on target. An excess of $20.9\pm 14.0$ events is observed in the energy range $475<E_{\nu }^{QE}<1250$ MeV, which, when constrained by the observed ${\bar{\nu }}_{\mu }$ events, has a probability for consistency with the background-only hypothesis of 0.5%. On the other hand, fitting for ${\bar{\nu }}_{\mu }\to {\bar{\nu }}_e$ oscillations, the best-fit point has a ${\chi }^2$-probability of 8.7%. The data are consistent with $\bar \nu_\mu \rightarrow \bar \nu_e$oscillationsinthe0.1to1.0eV$^2$ $\Delta m^2$ range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory.