Improved Search for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ Oscillations in the MiniBooNE Experiment

The MiniBooNE experiment at Fermilab reports results from an analysis of ${\overline{\nu }}_e$ appearance data from $11.27\times {10}^{20}$ protons on target in the antineutrino mode, an increase of approximately a factor of 2 over the previously reported results. An event excess of $78.4\pm 28.5$ events ($2.8\sigma$) is observed in the energy range $200<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$. If interpreted in a two-neutrino oscillation model, ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$, the best oscillation fit to the excess has a probability of 66% while the background-only fit has a ${\chi }^2$ probability of 0.5% relative to the best fit. The data are consistent with antineutrino oscillations in the $0.01<\Delta m^2<1.0{\mathrm{eV}}^2$ 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 $162.0\pm 47.8$ 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.

Figure 1

The antineutrino mode (top) and neutrino mode (bottom) $E_{\nu }^{\mathrm{QE}}$ distributions for ${\nu }_e$ CCQE data (points with statistical errors) and background (histogram with systematic errors).

Figure 2

The antineutrino mode (top) and neutrino mode (bottom) event excesses as a function of $E_{\nu }^{\mathrm{QE}}$. (Error bars include both the statistical and systematic uncertainties.) Also shown are the expectations from the best two-neutrino fit for each mode and for two example sets of oscillation parameters.

Figure 3

MiniBooNE allowed regions in the antineutrino mode (top) and the neutrino mode (bottom) for events with $E_{\nu }^{\mathrm{QE}}>200\mathrm{MeV}$ within a two-neutrino oscillation model. Also shown are the ICARUS [28] and KARMEN [24] appearance limits for neutrinos and antineutrinos, respectively. The shaded areas show the 90% and 99% C.L. LSND ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ allowed regions. The black stars show the MiniBooNE best fit points, while the circles show the example values used in Fig. 2.