Improved Search for $\bar ν_μ\rightarrow \bar ν_e$ Oscillations in the MiniBooNE Experiment

TL;DR

The MiniBooNE experiment at Fermilab reports results from an analysis of ν[over ¯](e) appearance data from 11.27×10(20) protons on target in the antineutrino mode, an increase of approximately a factor of 2 over the previously reported results.

Abstract

The MiniBooNE experiment at Fermilab reports results from an analysis of $\bar ν_e$ appearance data from $11.27 \times 10^{20}$ protons on target in antineutrino mode, an increase of approximately a factor of two over the previously reported results. An event excess of $78.4 \pm 28.5$ events ($2.8 σ$) is observed in the energy range $200<E_ν^{QE}<1250$ MeV. If interpreted in a two-neutrino oscillation model, $\barν_μ\rightarrow\barν_e$, 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 $0.01 < Δm^2 < 1.0$ eV$^2$ 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 $162.0 \pm 47.8$ events ($3.4 σ$) 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.

Figures (3)

• Figure 1: The antineutrino mode (top) and neutrino mode (bottom) $E_\nu^{QE}$ 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 $E_\nu^{QE}$. (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 antineutrino mode (top) and neutrino mode (bottom) for events with $E^{QE}_{\nu} > 200$ MeV within a two-neutrino oscillation model. Also shown are the ICARUS icarus and KARMEN karmen appearance limits for neutrinos and antineutrinos, respectively. The shaded areas show the 90% and 99% C.L. LSND $\bar{\nu}_{\mu}\rightarrow\bar{\nu}_e$ allowed regions. The black stars show the MiniBooNE best fit points, while the circles show the example values used in Fig. \ref{['excessnab']}.