Type II Seesaw leptogenesis

Abstract

The Type II Seesaw Mechanism provides a minimal framework to explain the neutrino masses involving the introduction of a single triplet Higgs to the Standard Model. However, this simple extension was believed to be unable to successfully explain the observed baryon asymmetry of the universe through Leptogenesis. In our previous work (Phys. Rev. Lett.128 (2022) 141801), we demonstrated that the triplet Higgs of the Type II Seesaw Mechanism alone can simultaneously generate the observed baryon asymmetry of the universe and the neutrino masses while playing a role in setting up Inflation. This is achievable with a triplet Higgs mass as low as 1 TeV, and predicts that the neutral component obtains a small vacuum expectation value v∆< 10 keV. We find that our model has very rich phenomenology and can be tested by various terrestrial experiments as well as by astronomical observations. Particularly, we show that the successful parameter region may be probed at a future 100 TeV collider, upcoming lepton flavor violation experiments such as Mu3e, and neutrinoless double beta decay experiments. Additionally, the tensor-to-scalar ratio from the inflationary scenario will be probed by the LiteBIRD telescope, and observable isocurvature perturbations may be produced for some parameter choices. In this article, we present all the technical details of our calculations and further discussion of its phenomenological implications.

The Type II Dirac Seesaw Portal to the mirror sector: Connecting neutrino masses and a solution to the strong CP problem

“…Since there is no direct interaction coupling baryons to mirror baryons, the respective asymmetries will not equilibrate to zero and remain separately conserved. The authors of [123,124] put forth a very economical framework unifying Higgs inflation [125] and the conventional Type II Seesaw, where the coherent rotation of the complex triplet in field space during inflation facilitates leptogenesis via the Affleck-Dine mechanism [126]. In this picture, the Sakharov conditions [127] are realized via the initial phase of the triplet field providing C and CP violation, deviations from thermal equilibrium appear during cosmic inflation and baryon number violation arises from lepton number violation in the scalar potential, that is transmitted to the SM leptons and converted into baryon number via the B+L violating sphaleron vertex.…”

“…In this picture, the Sakharov conditions [127] are realized via the initial phase of the triplet field providing C and CP violation, deviations from thermal equilibrium appear during cosmic inflation and baryon number violation arises from lepton number violation in the scalar potential, that is transmitted to the SM leptons and converted into baryon number via the B+L violating sphaleron vertex. Analogous to [123,124], we will assume that the inflaton is a linear combination of the neutral fields h, h ′ and h Φ + ia Φ ≡ ρ Φ e iφ after giving all scalar multiplets a non-minimal coupling to gravity [128]. Following the discussion at the end of section II, we assign the B-L charge of two to Φ and treat the κHΦ † H ′ term as an explicit B-L breaking by two units.…”

“…Since the field value of the inflaton approaches the Planck scale during inflation, the trilinear scalar term is subdominant compared to other Planck-scale suppressed effective operators and will only matter when the field value has decreased due to the cosmic expansion. Even worse, during reheating, the trilinear coupling can lead to oscillations of the scalar condensate instead of a rotation, manifesting as an oscillation in the lepton asymmetry spoiling the mechanism unless we [123,124] |κ| < 10 −18 M Pl. ≃ O(10 GeV).…”

“…Additionally, for the asymmetry generation, an operator of dimension larger than four is needed so that the produced lepton number is conserved during reheating [123,124]. Consequently, we consider the following dimension five operator…”

“…Exploring this idea and the full treatment of the inflationary dynamics will be left for future work. In the following we stick to the symmetry-conserving case and use plus signs following [123,124] so that the dimension five term is ∝ cos(φ). Up to mixing angles between the scalars during inflation and order one factors the lepton asymmetry at the end of inflation turns out to be [123,124] n L end ≃ −2 λ 5 ρ 3 end sin (φ 0 ) 3 λ ,…”