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We develop an extension of the basic inverse seesaw model which addressessimultaneously two of its drawbacks, namely, the lack of explanation of thetiny Majorana mass term $\mu$ for the TeV-scale singlet fermions and thedifficulty in achieving successful leptogenesis. Firstly, we investigatesystematically leptogenesis within the inverse (and the related linear) seesawmodels and show that a successful scenario requires either small Yukawacouplings, implying loss of experimental signals, and/or quasi-degeneracy amongsinglets mass of different generations, suggesting extra structure must beinvoked. Then we move to the analysis of our new framework, which we refer toas hybrid seesaw. This combines the TeV degrees of freedom of the inverseseesaw with those of a high-scale ($M_N\gg$ TeV) seesaw module in such a way asto retain the main features of both pictures: naturally small neutrino masses,successful leptogenesis, and accessible experimental signatures. We show howthe required structure can arise from a more fundamental theory with a gaugesymmetry or from warped extra dimensions/composite Higgs. We provide a detailedderivation of all the analytical formulae necessary to analyze leptogenesis inthis new framework, and discuss the entire gamut of possibilities our scenarioencompasses: including scenarios with singlet masses in the enlarged range $M_N\sim 10^6 - 10^{16}$ GeV. The idea of hybrid seesaw was proposed by us inarXiv:1804.06847; here, we substantially elaborate upon and extend earlierresults.

Natural Seesaw and Leptogenesis from Hybrid of High-Scale Type I and TeV-Scale Inverse