Search for sterile neutrino mixing in the MINOS long-baseline experiment

A search for depletion of the combined flux of active neutrinospecies over a 735 km baseline is reported using neutral-currentinteraction data recorded by the MINOSdetectors in the NuMI neutrinobeam. Such a depletion is not expected according to conventional interpretations of neutrino oscillationdata involving the three known neutrinoflavors. A depletion would be a signature of oscillations or decay to postulated noninteracting sterile neutrinos, scenarios not ruled out by existing data. From an exposure of 3.18×10^(20) protons on target in which neutrinosof energies between ∼500  MeV and 120 GeV are produced predominantly as ν_μ, the visible energy spectrum of candidate neutral-currentreactions in the MINOSfar detector is reconstructed. Comparison of this spectrum to that inferred from a similarly selected near-detector sample shows that of the portion of the ν_μ flux observed to disappear in charged-currentinteraction data, the fraction that could be converting to a sterile state is less than 52% at 90% confidence level (C.L.). The hypothesis that active neutrinosmix with a single sterile neutrinovia oscillations is tested by fitting the data to various models. In the particular four- neutrinomodels considered, the mixing angles θ_(24) and θ_(34) are constrained to be less than 11° and 56° at 90% C.L., respectively. The possibility that active neutrinosmay decay to sterile neutrinosis also investigated. Pure neutrinodecay without oscillations is ruled out at 5.4 standard deviations. For the scenario in which active neutrinosdecay into sterile states concurrently with neutrino oscillations, a lower limit is established for the neutrinodecay lifetime τ_3/m_3>2.1×10^(-12) s/eV at 90% C.L.