Solar and atmospheric four-neutrino oscillations

We present an analysis of the neutrino oscillation solutions of the solar and atmospheric neutrino problems in the framework of four-neutrino mixing where a sterile neutrino is added to the three standard ones and the mass spectra present two separated doublets. Such scenarios allow for simultaneous transitions of solar ν_e, as well as atmospheric ν_μ, into active and sterile neutrinos controlled by the additional mixing angles θ₂₃ and θ₂₄, and they contain as limiting cases the pure solar ν_e-active and ν_e-sterile neutrino oscillations, and the pure atmospheric ν_μ→ν_s and ν_μ→ν_τ oscillations, respectively. We evaluate the allowed active-sterile admixture in both solar and atmospheric oscillations from the combined analysis. Our results show that, although the Super-Kamiokande data disfavor both the pure ν_μ→ν_s atmospheric channel and the pure ν_e→ν_s solar channel, the result from the combined analysis still favors close-to-pure active and sterile oscillations and disfavors oscillations into a near-maximal active-sterile admixture.