Generalized inverse seesaw mechanisms

Abstract

The seesaw mechanism can be generalized to a type-III variant and a quintuplet variant. We present two models that provide analogous generalizations of the inverse seesaw mechanism. The first model employs a real fermion triplet $\mathcal{F}\ensuremath{\sim}(1,3,0)$ and requires no additional multiplets or parameters relative to the standard inverse seesaw. We argue that, from a bottom-up perspective, there appears to be no particular reason to preference the usual scenario over this variant. The second model employs a fermion quintuplet $\mathcal{F}\ensuremath{\sim}(1,5,0)$ and requires an additional scalar $S\ensuremath{\sim}(1,4,1)$. We also show that minimal inverse seesaws with even larger fermionic representations are not expected to realize naturally small neutrino masses.