Abstract
The scotogenic model proposed by Ernest Ma represents an attractive and minimal example for the generation of small Standard Model neutrino masses via radiative corrections in the dark matter sector. In this paper, we demonstrate that, in addition to neutrino masses and dark matter, the scotogenic model also allows to explain the baryon asymmetry of the Universe via low-scale leptogenesis. First, we consider the case of two right-handed neutrinos (RHNs) N_{1,2}, for which we provide an analytical argument why it is impossible to push the RHN mass scale below M_1^min ~ 10^10 GeV, which is identical to the value in standard thermal leptogenesis in the type-I seesaw scenario with the same washout strength. Then, we present a detailed study of the three-RHN case based on both an analytical and a numerical analysis. In the case of three RHNs, we obtain a lower bound on the N_1 mass of around 10 TeV. Remarkably enough, successful low-scale leptogenesis can be achieved without any degeneracy in the RHN mass spectrum. The only necessary condition is a suppression in the N_1 Yukawa couplings, which results in suppressed washout and a small active neutrino mass of around 10^-12 eV. This leads to the fascinating realization that low-scale leptogenesis in the scotogenic model can be tested in experiments that aim at measuring the absolute neutrino mass scale.
Highlights
The baryon asymmetry of the Universe (BAU)— conventionally quantified in terms of the cosmic baryonto-photon ratio ηoBbs ≃ 6.1 × 10−10 [1,2]—cannot be explained within the Standard Model (SM) of particle physics
In its standard formulation, leptogenesis is closely related to the type-I seesaw mechanism [4,5,6,7,8] that aims at explaining the small SM neutrino masses by introducing two or more sterile right-handed neutrinos (RHNs) Ni with large Majorana masses Mi
Enough, the matrix h†h is independent of the PMNS matrix U. This indicates that the CP-violating phases relevant for leptogenesis are independent of the CP-violating phases in the PMNS matrix
Summary
The baryon asymmetry of the Universe (BAU)— conventionally quantified in terms of the cosmic baryonto-photon ratio ηoBbs ≃ 6.1 × 10−10 [1,2]—cannot be explained within the Standard Model (SM) of particle physics. A future detection of lepton number violation at low energies may readily rule out high-scale leptogenesis altogether [21,22,23] Taken together, these observations serve as a motivation to seek alternatives to the paradigm of standard thermal leptogenesis in the type-I. seesaw model that manage to generate the BAU at a (much) lower RHN mass scale. The main purpose of this paper is to present a promising example for such a low-scale alternative to standard thermal leptogenesis To this end, we will carry out an in-depth study of thermal leptogenesis in Ernest Ma’s scotogenic model of radiative neutrino masses [24]. [51] studied thermal leptogenesis in two-Higgs-doublet models from a more general perspective It properly accounted for the low-energy neutrino data, but only provided a few analytical estimates and refrained from solving the corresponding set of Boltzmann equations.
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