Improved Search for $\bar \nu_\mu \rightarrow \bar \nu_e$ Oscillations in the MiniBooNE Experiment

TL;DR

The MiniBooNE experiment observed an excess of electron antineutrino events suggesting possible oscillations, with increased data providing stronger evidence and constraining background explanations, indicating potential new physics beyond the standard model.

Contribution

This paper presents the most extensive antineutrino oscillation analysis from MiniBooNE, providing new evidence for neutrino oscillations and exploring implications for sterile neutrino models.

Findings

Observed 78.4 ± 28.5 excess events in antineutrino mode.

Data favor oscillation hypothesis with 66% probability.

Results are consistent with oscillations in the 0.01–1.0 eV² Δm² range.

Abstract

The MiniBooNE experiment at Fermilab reports results from an analysis of $\bar \nu_e$ 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 ($2.8 \sigma$) is observed in the energy range $200<E_\nu^{QE}<1250$ MeV. If interpreted in a two-neutrino oscillation model, $\bar{\nu}_{\mu}\rightarrow\bar{\nu}_e$, 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 $0.01 < \Delta m^2 < 1.0$ 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 ($3.4 \sigma$) 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.