Remark on Natural Models of Neutrinos

We comment on what the naturalness argument of 't Hooft implies for a minimalextension of the standard model which incorporates right-handed neutrinos withgeneric mass terms. If this Lagrangian is taken as a low energy effectivetheory, the idea of pseudo-Dirac neutrinos with very small masses is consistentwith the naturalness argument of 't Hooft. This argument is based on anobservation that the right-handed components of neutrinos in the massless limitexhibit an extra enhanced symmetry which is absent in other charged fermions.This enhanced symmetry is reminiscent of the Nambu-Goldstone fermionsassociated with spontaneously broken supersymmetry. The conventional seesawscenario gives another natural solution if the ultra-heavy right-handedneutrinos are integrated out in defining a low energy effective theory.Comment: 13 pages. Modified the title, abstract and the body of the paper. To be published in Prog. Theor. Phy