Normal vs. Inverted Hierarchy in Type I Seesaw Models

Abstract

We demonstrate that, for every grand unified model based on a conventional type I seesaw mechanism leading to a normal light neutrino mass hierarchy, one can easily generate a corresponding model with an inverted hierarchy which yields the same neutrino oscillation parameters. However, the latter type model has several unattractive instabilities which will disfavor any grand unified type I seesaw model, if an inverted neutrino mass hierarchy is observed experimentally. This should be contrasted with the softly-broken $L_e-L_{\mu }-L_{\tau }$ flavor symmetry models which are eliminated, if the data favors a normal mass hierarchy.

Comments: 10 pages including 1 figure; improved version with additional illustration of results for whole class of models with charged lepton mass matrices diagonal in flavor space; accepted for publication in Physics Letters B