Improved Search for ν¯µ → ν¯e Oscillations in the MiniBooNE Experiment

The MiniBooNE experiment at Fermilab reports results from an analysis of ¯νe appearance data from 11.27 × 10²⁰ protons on target in antineutrino mode, an increase of approximately a factor of two over the previously reported results. An event excess of 78.4 ± 28.5 events (2.8σ) is observed in the energy range 200 < EQE ν < 1250 MeV. If interpreted in a two-neutrino oscillation model, ν¯µ → ν¯e, the best oscillation fit to the excess has a probability of 66% while the background-only fit has a χ 2 -probability of 0.5% relative to the best fit. The data are consistent with antineutrino oscillations in the 0.01 < ∆m² < 1.0 eV² range and have some overlap with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector (LSND). All of the major backgrounds are constrained by in-situ event measurements so non-oscillation 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 (3.4σ) 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.