Event Excess in the MiniBooNE Search for $\bar ν_μ\rightarrow \bar ν_e$ Oscillations

TL;DR

The MiniBooNE experiment at Fermilab reports results from a search for ¯ν_{μ}→¯ν_{e} oscillations, using a data sample corresponding to 5.66×10²⁰ protons on target, consistent with antineutrino oscillations in the 0.1 to 1.0  eV² Δm² range.

Abstract

The MiniBooNE experiment at Fermilab reports results from a search for $\bar ν_μ\rightarrow \bar ν_e$ 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 $475<E_ν^{QE}<1250$ MeV, which, when constrained by the observed $\bar ν_μ$ events, has a probability for consistency with the background-only hypothesis of 0.5\%. On the other hand, fitting for $\barν_μ\rightarrow\barν_e$ oscillations, the best-fit point has a $χ^2$-probability of 8.7\%. The data are consistent with $\bar ν_μ\rightarrow \bar ν_e$ oscillations in the 0.1 to 1.0 eV$^2$ $Δm^2$ range and with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory.

Figures (5)

• Figure 1: Top: The $E_\nu^{QE}$ distribution for $\bar{\nu}_e$ CCQE data (points with statistical errors) and background (histogram with systematic errors). Bottom: The event excess as a function of $E_\nu^{QE}$. Also shown are the expectations from the best oscillation fit with $E_\nu^{QE}>475$ MeV, $(\Delta m^2, \sin^2 2 \theta)$ = (0.064 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 $E_{vis}$ (top panel) and $\cos(\theta)$ (bottom panel) distributions for data (points with statistical errors) and backgrounds (histogram with systematic errors) for $E_\nu^{QE} > 200$ MeV.
• Figure 3: MiniBooNE 68%, 90%, and 99% C.L. allowed regions for events with $E^{QE}_{\nu} > 475$ MeV within a two neutrino $\bar{\nu}_{\mu}\rightarrow\bar{\nu}_e$ oscillation model. Also shown are limits from KARMEN karmen and Bugey bugey. 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 $E^{QE}_{\nu} > 200$ MeV within a two neutrino $\bar{\nu}_{\mu}\rightarrow\bar{\nu}_e$ oscillation model. The solid (dashed) curves are without (with) the subtraction of the expected 12 event excess in the $200<E_\nu^{QE}<475$ MeV low-energy region from the neutrino component of the beam. Also shown are limits from KARMEN karmen and Bugey bugey. 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 $L/E_\nu^{QE}$ for $\bar{\nu}_\mu \rightarrow \bar{\nu}_e$ candidate events from MiniBooNE and LSND. The data points include both statistical and systematic errors.