Evidence for Neutrino Oscillations from the Observation of Electron Anti-neutrinos in a Muon Anti-Neutrino Beam

TL;DR

The LSND study reports evidence for neutrino flavor oscillations driven by muon decay-at-rest neutrinos, identifying an excess of $\bar{\nu}_e p \rightarrow e^+ n$ events that cannot be explained by conventional backgrounds. Using a sophisticated likelihood fit that incorporates both DAR and DIF channels and a robust background-constraining program, the analysis finds an oscillation probability of $(0.264\pm0.067\pm0.045)\%$, compatible with a two-flavor interpretation in the range $\Delta m^2 \sim 0.2-10\ \mathrm{eV}^2/c^4$ and requiring a neutrino mass $>0.4\ \mathrm{eV}/c^2$. The final result combines six years of data with an improved reconstruction that enhances the $e^+$–$\gamma$ correlation from neutron capture, and yields consistent findings with earlier analyses while extending the inferred allowed region. A decay-in-flight (DIF) analysis shows no definitive excess but remains compatible with higher-precision DAR results; the paper discusses methods to manage statistical and systematic uncertainties and provides a global fit that describes multiple neutrino processes. The LSND findings imply the need for physics beyond the three-neutrino paradigm (e.g., sterile neutrinos) and motivate the MiniBooNE program to perform an independent test of the anomalous appearance signal. Overall, the work constitutes a landmark short-baseline oscillation measurement with significant implications for neutrino mass and mixing, and for constraining beyond-Standard-Model scenarios in the neutrino sector.

Abstract

A search for muon anti-neutrino to electron anti-neutrino oscillations was conducted by the Liquid Scintillator Neutrino Detector at the Los Alamos Neutron Science Center using muon anti-neutrinos from positive muon decay at rest. A total excess of 87.9 +/- 22.4 +/- 6.0 events consistent with electron anti-neutrino plus proton scattering to positron plus neutron was observed above the expected background. This excess corresponds to an oscillation probability of (0.264 +/- 0.067 +/- 0.045), which is consistent with an earlier analysis. In conjunction with other known limits on neutrino oscillations, the LSND data suggest that neutrino oscillations occur in the 0.2-10 eV^2/c^4 Delta-m^2 range, indicating a neutrino mass greater than 0.4 eV/c^2.

Figures (28)

• Figure 1: The layout of the LSND detector and the A6 beam stop area.
• Figure 2: The layout of the A6 beam stop, as it was configured for the 1993-1995 data taking.
• Figure 3: The decay-at-rest neutrino fluxes averaged over the detector.
• Figure 4: The decay-in-flight neutrino fluxes averaged over the detector.
• Figure 5: The electron and $\beta$ energy distributions and the time between the electron and $\beta$, $\Delta t$, for $^{12}C(\nu_e,e^-)^{12}N_{g.s.}$ scattering events.