| Summary: | 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.
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