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

The MiniBooNE experiment at Fermilab reports results from a search for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ 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 $475<E_{\nu }^{\mathrm{QE}}<1250\mathrm{MeV}$, which, when constrained by the observed ${\overline{\nu }}_{\mu }$ events, has a probability for consistency with the background-only hypothesis of 0.5%. On the other hand, fitting for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillations, the best-fit point has a ${\chi }^2$ probability of 8.7%. The data are consistent with ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillations in the 0.1 to $1.0{\mathrm{eV}}^2$ $\Delta m^2$ range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory.

Figure 1

Top: The $E_{\nu }^{\mathrm{QE}}$ distribution for ${\overline{\nu }}_e$ CCQE data (points with statistical errors) and background (histogram with systematic errors). Bottom: The event excess as a function of $E_{\nu }^{\mathrm{QE}}$. Also shown are the expectations from the best oscillation fit with $E_{\nu }^{\mathrm{QE}}>475\mathrm{MeV}$, $(\Delta m^2,{sin}^{2}2\theta )=(0.064{\mathrm{eV}}^2,0.96)$, where the fit is extrapolated below 475 MeV, and from two other oscillation parameter sets in the allowed region. No correction has been made for the low-energy excess of events seen in neutrino mode below 475 MeV. All known systematic errors are included in the systematic error estimate.

Figure 2

The $E_{\mathrm{vis}}$ (top panel) and $\cos (\theta )$ (bottom panel) distributions for data (points with statistical errors) and backgrounds (histogram with systematic errors) for $E_{\nu }^{\mathrm{QE}}>200\mathrm{MeV}$.

Figure 3

MiniBooNE 68%, 90%, and 99% C.L. allowed regions for events with $E_{\nu }^{\mathrm{QE}}>475\mathrm{MeV}$ within a two-neutrino ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillation model. Also shown are limits from KARMEN [15] and Bugey [25]. The Bugey curve is a 1-sided limit for ${sin}^{2}2\theta$ corresponding to $\Delta {\chi }^2=1.64$, while the KARMEN curve is a “unified approach” 2D contour. The shaded areas show the 90% and 99% C.L. LSND allowed regions. The black dot shows the best-fit point.

Figure 4

MiniBooNE 90% and 99% C.L. allowed regions for events with $E_{\nu }^{\mathrm{QE}}>200\mathrm{MeV}$ within a two-neutrino ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ oscillation model. The solid (dashed) curves are without (with) the subtraction of the expected 12 event excess in the $200<E_{\nu }^{\mathrm{QE}}<475\mathrm{MeV}$ low-energy region from the neutrino component of the beam. Also shown are limits from KARMEN [15] and Bugey [25]. The shaded areas show the 90% and 99% C.L. LSND allowed regions. The black dots show the best-fit points.

Figure 5

The oscillation probability as a function of $L/E_{\nu }^{\mathrm{QE}}$ for ${\overline{\nu }}_{\mu }\to {\overline{\nu }}_e$ candidate events from MiniBooNE and LSND. The data points include both statistical and systematic errors.