Resolution of puzzles from the LSND, KARMEN, and MiniBooNE experiments

Abstract

This work has attempted to reconcile puzzling neutrino oscillation results from the LSND, KARMEN and MiniBooNE experiments. We show that the LSND evidence for $\barν_μ\to \barν_e$ oscillations, its long-standing disagreement with the results from KARMEN, and the anomalous event excess observed by MiniBooNE in $ν_μ$ and $\barν_μ$ data could all be explained by the existence of a heavy sterile neutrino ($ν_h$). All these results are found to be consistent with each other assuming that the $ν_h$ is created in $ν_μ$ neutral-current interactions and decays radiatively into a photon and a light neutrino. Assuming the $ν_h$ is produced through mixing with $ν_μ$, the combined analysis of the LSND and MiniBooNe excess events suggests that the $ν_h$ mass is in the range from 40 to 80 MeV, the mixing strength is $|U_{μh}|^2 \simeq 10^{-3}-10^{-2}$, and the lifetime is $τ_{ν_h} \lesssim 10^{-9}$ s. Surprisingly, this LSND-MiniBooNE parameters window is found to be unconstrained by the results from the most sensitive experiments searching for heavy neutrino. We set new limits on $|U_{μh}|^2$ for the LSND-MiniBooNE favorable mass region from the precision measurements of the Michel spectrum by the TWIST experiment. The results obtained provide a strong motivation for a sensitive search for the $ν_h$ in a near future $ K$ decay or neutrino experiments, which fit well in the existing/planned experimental programs at CERN or FNAL. The question of whether the heavy neutrino is Dirac or Majorana particle is briefly discussed.