Improved Search for (nu)over-bar(mu) -> (nu)over-bar(e) Oscillations in the MiniBooNE Experiment

The MiniBooNE experiment at Fermilab reports results from an analysis of (nu)overbar(e) appearance data from 11.27 X 10(20) protons on target in the antineutrino mode, an increase of approximately a factor of 2 over the previously reported results. An event excess of 78.4 +/- 28.5 events (2.8 sigma) is observed in the energy range 200 < E-nu(QE) < 1250 MeV. If interpreted in a two-neutrino oscillation model, <(nu)over bar>(mu) -> (nu) over bar (e), the best oscillation fit to the excess has a probability of 66% while the background-only fit has chi(2) probability of 0.5% relative to the best fit. The data are consistent with antineutrino oscillations in the 0.01 < Delta m(2) < 1.0 eV(2) range and have some overlap with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector. All of the major backgrounds are constrained by in situ event measurements so nonoscillation explanations would need to invoke new anomalous background processes. The neutrino mode running also shows an excess at low energy of 162.0 +/- 47.8 events (3.4 sigma) but the energy distribution of the excess is marginally compatible with a simple two neutrino oscillation formalism. Expanded models with several sterile neutrinos can reduce the incompatibility by allowing for CP violating effects between neutrino and antineutrino oscillations.