Remark on Natural Models of Neutrinos
  We comment on what the naturalness argument of 't Hooft implies for a minimal extension of the standard model which incorporates right-handed neutrinos with generic mass terms. If this Lagrangian is taken as a low energy effective theory, the idea of pseudo-Dirac neutrinos with very small masses is consistent with the naturalness argument of 't Hooft. This argument is based on an observation that the right-handed components of neutrinos in the massless limit exhibit an extra enhanced symmetry which is absent in other charged fermions. This enhanced symmetry is reminiscent of the Nambu-Goldstone fermions associated with spontaneously broken supersymmetry. The conventional seesaw scenario gives another natural solution if the ultra-heavy right-handed neutrinos are integrated out in defining a low energy effective theory. 
Kazuo Fujikawa