Abstract
In the simplest type-I seesaw leptogenesis scenario right-handed neutrino annihilation processes are absent. However, in the presence of new interactions these processes are possible and can affect the resulting B-L asymmetry in an important way. A prominent example is provided by models with spontaneous lepton number violation, where the existence of new dynamical degrees of freedom can play a crucial role. In this context, we provide a model-independent discussion of the effects of right-handed neutrino annihilations. We show that in the weak washout regime, as long as the scattering processes remain slow compared with the Hubble expansion rate throughout the relevant temperature range, the efficiency can be largely enhanced, reaching in some cases maximal values. Moreover, the B-L asymmetry yield turns out to be independent upon initial conditions, in contrast to the ''standard'' case. On the other hand, when the annihilation processes are fast, the right-handed neutrino distribution tends to a thermal one down to low temperatures, implying a drastic suppression of the efficiency which in some cases can render the B-L generation mechanism inoperative.
Highlights
The only indications for physics beyond the SU(2)L ⊗ U(1)Y model come from the lepton sector and cosmology
Seesaw schemes with spontaneous lepton number violation generate neutrino mass while potentially addressing other cosmological issues such as the origin of dark matter and inflation. They can account for the cosmic baryon asymmetry via leptogenesis
We have identified the two new parameters which best describe the implications of such scatterings in the generation of the B − L asymmetry
Summary
The only indications for physics beyond the SU(2)L ⊗ U(1)Y model come from the lepton sector and cosmology. If the thermal bath is populated with the new scalars before the leptogenesis era [32], these processes can efficiently populate the plasma with RH neutrinos, basically resembling what happens in scalar and fermion triplet leptogenesis models [33,34,35,36] If this turns out to be the case, soon after these processes become active the heat bath can be readily populated with a RH neutrino thermal distribution, implying that if at early times the 2 ↔ 2 scattering processes and the RH neutrinos inverse decays are frozen, a sizeable enhancement of the B − L yield can be expected. Motivated by type-I seesaw models with spontaneous lepton number violation, these effects in seesaw leptogenesis are not restricted to majoron models They will be present in all models with extended scalar sectors that couple to the RH neutrinos.
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