Neutrino Oscillation based on the Mixings with a Heavy Right-Handed   Neutrino
  We study a consistent explanation for both deficits of solar neutrino and atmospheric neutrino due to the neutrino oscillation induced by the mixing of light neutrinos with a heavy right-handed neutrino. We propose such a phenomenological neutrino mass matrix that realizes this scenario in the simple three generation left- and right- handed neutrino framework. Although this model contains only one nonzero mass eigenvalue $\sim$ 10 eV for these light neutrino states at the first approximation level, it can be expected to explain consistently both deficiency due to the appropriate higher order corrections. A suitable hot dark matter candidate is naturally included in it as its own feature. 
Daijiro Suematsu