Evidence for nu_mu -> nu_e Oscillations from Pion Decay in Flight Neutrinos

TL;DR

This paper reports a search for nu_mu -> nu_e oscillations using pion decay in flight neutrinos at LSND, providing a complementary test to the LSND DAR result. Two independent analyses identify a beam-on excess in nu_e-like events after subtracting beam-unrelated backgrounds, with an oscillation probability around 2.6 × 10^-3. Systematic uncertainties from flux, cross sections, and electron identification are rigorously assessed to produce confidence regions in sin^2 2θ–Δm^2 that align with the prior LSND findings. Together, the results offer independent support for neutrino oscillations in a DIF beam, with distinct backgrounds and systematics from the DAR channel, reinforcing the oscillation hypothesis.

Abstract

A search for nu_mu -> nu_e oscillations has been conducted at the Los Alamos Meson Physics Facility using nu_mu from pi^+ decay in flight. An excess in the number of beam-related events from the C(nu_e,e^-)X inclusive reaction is observed. The excess is too large to be explained by normal nu_e contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of (2.6 +- 1.0 +- 0.5) x 10^{-3} is consistent with the previously reported nu_mu_bar -> nu_e_bar oscillation evidence from LSND.

Figures (32)

• Figure 1: The LSND $(\sin^2 2\theta,\Delta m^2)$ favored regions obtained from the $\bar{\nu}_\mu \to \bar{\nu}_e$ DAR oscillations search. The darkly-shaded and lightly-shaded regions correspond to 90% and 99% likelihood regions. Also shown are the 90% confidence level limits from KARMEN (dashed), E776 (dotted) and the Bugey reactor experiment (dot-dashed).
• Figure 2: Calculated $\nu_\mu$ and $\nu_e$ DIF fluxes at the detector center from the A6 target (solid histograms) and from the A1+A2 targets (dashed histograms).
• Figure 3: Time difference distribution to events subsequent to all primary events for the initial DIF data after standard PID, veto shield hit multiplicity and fiducial volume cuts.
• Figure 4: Reconstructed distance distribution between Michel electron and parent stopped cosmic-ray muon, versus muon tank hit multiplicity. Figure (a) is for muons with a veto shield hit multiplicity $\ge$ 6 and (b) is for muons with a veto shield hit multiplicity $<$ 6. The regions in the upper left corners, delimited by the dashed lines, are the regions allowed by this selection (see text).
• Figure 5: (a) Time difference to future events and (b) energy distribution of the future events for primary events of the initial DIF sample that fail the future space-time correlations. The fit in (a) is to an exponential plus a constant with a time constant of 2.2 $\mu s$. The fit in (b) is to the Michel electron spectrum shape.