Abstract
We develop an extension of the basic inverse seesaw model which addresses simultaneously two of its drawbacks, namely, the lack of explanation of the tiny Majorana mass term μ for the TeV-scale singlet fermions and the difficulty in achieving successful leptogenesis. Firstly, we investigate systematically leptogenesis within the inverse (and the related linear) seesaw models and show that a successful scenario requires either small Yukawa couplings, implying loss of experimental signals, and/or quasi-degeneracy among singlets mass of different generations, suggesting extra structure must be invoked. Then we move to the analysis of our new framework, which we refer to as hybrid seesaw. This combines the TeV degrees of freedom of the inverse seesaw with those of a high-scale (MN ≫ TeV) seesaw module in such a way as to retain the main features of both pictures: naturally small neutrino masses, successful leptogenesis, and accessible experimental signatures. We show how the required structure can arise from a more fundamental theory with a gauge symmetry or from warped extra dimensions/composite Higgs. We provide a detailed derivation of all the analytical formulae necessary to analyze leptogenesis in this new framework, and discuss the entire gamut of possibilities our scenario encompasses — including scenarios with singlet masses in the enlarged range MN ∼ 106 − 1016 GeV. This idea of hybrid seesaw was proposed by us in arXiv:1804.06847; here, we substantially elaborate upon and extend earlier results.
Highlights
Viewing the Standard Model (SM) as an effective field theory, Majorana neutrino masses mν dominantly arise from the unique dimension-five Weinberg operator: C y2 mNP H → mν = y2v2 mNP (1.1)where and H are respectively the SM lepton and Higgs doublets with vacuum expectation value (VEV) v
An analogous calculation for the linear seesaw (LSS) model is shown in appendix A and the parametric dependences of the final baryon asymmetry of the two seesaw models are the same, as we summarize in table 2
While the present paper focuses on leptogenesis from decays of singlets, the lepton asymmetry can be realized via flavor oscillation among singlets, as first pointed out by Akhmedov, Rubakov and Smirnov (ARS) [46]
Summary
The new degrees of freedom responsible for generating the operator in eq (1.1) are assumed to be characterized by a mass scale mNP and a leading coupling y to the SM lepton (and the Higgs) (couplings among new states are measured by other couplings in general). The operator in eq (1.1) violates U(1)B−L by two units Such a violation may be induced directly from y2/mNP, as in ordinary type I seesaw scenarios [1,2,3,4,5]. In all those cases we conventionally say U(1)B−L breaking is maximal and set C ≡ 1 to mean that no further parameter is necessary to generate neutrino masses. We will refer to these models as scenarios with small
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