Atmospheric neutrinos, ν e - ν s oscillations and a novel neutrino evolution equation

If a sterile neutrino ν s with an ev-scale mass and a sizeable mixing to the electron neutrino exists, as indicated by the reactor and gallium neutrino anomalies, a strong resonance enhancement of ν e -ν s oscillations of atmospheric neutrinos should occur in the tev energy range. at these energies neutrino flavour transitions in the 3+1 scheme depend on just one neutrino mass squared difference and are fully described within a 3-flavour oscillation framework. we demonstrate that the flavour transitions of atmospheric ν e can actually be very accurately described in a 2-flavour framework, with neutrino flavour evolution governed by an inhomogeneous schrödinger-like equation. evolution equations of this type have not been previously considered in the theory of neutrino oscillations.