Abstract
Low-scale leptogenesis is most efficient in the limit of an extreme mass degeneracy of right-handed neutrino flavours. Two variants of this situation are of particular interest: large neutrino Yukawa couplings, which boost the prospects of experimental scrutiny, and small ones, which may lead to large lepton asymmetries surviving down to T < 5 GeV. We study benchmarks of these cases within a “complete” framework which tracks both helicity states of right-handed neutrinos as well as their kinetic non-equilibrium, and includes a number of effects not accounted for previously. For two right-handed flavours with GeV-scale masses, Yukawa couplings up to |h| ∼ 0.7×10−5 are found to be viable for baryogenesis, with ΔM/M ∼ 10−8 as the optimal degeneracy. Late-time lepton asymmetries are most favourably produced with ΔM/M ∼ 10−11. We show that the system reaches a stationary state at T < 15 GeV, in which lepton asymmetries can be more than 103 times larger than the baryon asymmetry, reach flavour equilibrium, and balance against helicity asymmetries.
Highlights
An extension of the Standard Model through two or three generations of right-handed neutrinos, which account for the observed active neutrino mass differences and mixings, offers for a simple explanation of the baryon asymmetry in the present universe [1]
We show that the system reaches a stationary state at T < 15 GeV, in which lepton asymmetries can be more than 103 times larger than the baryon asymmetry, reach flavour equilibrium, and balance against helicity asymmetries
On the other hand late-time lepton asymmetries can be considerably larger than the baryon asymmetry, but are obtained preferably with small values of Im z and a more extreme degeneracy around ∆M/M ∼ 10−11, so that leptogenesis takes place as late as possible
Summary
An extension of the Standard Model through two or three generations of right-handed neutrinos, which account for the observed active neutrino mass differences and mixings, offers for a simple explanation of the baryon asymmetry in the present universe [1]. If the Majorana masses are assumed to be “hierarchical”, only the lightest among them plays a substantial role in leptogenesis. This prototypical example has been studied to great detail including the effect of radiative corrections The equilibration rates contain both “direct” and “indirect” contributions, with the former referring to 1 ↔ 2 and 2 ↔ 2 decays or scatterings and the latter to rates experienced by off-shell left-handed neutrinos, which subsequently “oscillate” into right-handed neutrinos thanks to the presence of the Higgs mechanism at T
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