Constraints on neutrino masses from leptogenesis models

Abstract

Upper bounds on the CP asymmetry relevant for leptogenesis are reexamined and found weaker than in previous literature, both for hierarchical and for quasi-degenerate right-handed neutrinos. Successful leptogenesis implies the usual lower bound on right-handed neutrino masses, and an upper bound on left-handed neutrino masses (which we obtain to be 0.15 eV at 3sigma) only if right-handed neutrinos are assumed to be much more hierarchical than left-handed neutrinos. Other-wise both bounds can be considerably relaxed. The constraint on light neutrino masses varies assuming different interpretations of why neutrinos should be quasi-degenerate. With conservative assumptions, we find that a mild quasi-degeneracy allows neutrinos heavier than an eV compatibly with leptogenesis. We also extend computations of thermal leptogenesis to an alternative model of neutrino mass mediated by fermion triplets which was never considered so far for leptogenesis. Leptogenesis can be successful despite the effect of gauge interactions, resulting in only slightly stronger constraints on neutrino masses. (C) 2004 Published by Elsevier B.V

Right-handed sector leptogenesis

“…Also this mechanism can lead to successful leptogenesis in agreement with the neutrino mass constraints [7,8,9]. Other seesaw possibilities of successful leptogenesis arise if there are two or more heavy Higgs triplets [10,11] or if self-conjugate triplets of fermions Σ exist [12].…”

“…(11) and (12). At a lower scale, the light neutrino mass constraints generically require that the Y N couplings are smaller than 10 −3 , barring cancellations between different Y N entries.…”

Sommerfeld corrections to type-II and type-III leptogenesis

“…Following [3,4] the final baryon asymmetry is parameterized as n B /n γ = −0.029ηε, where the efficiency η encodes the dynamics and ε the amount of CP violation. The Boltzmann equation that dictates the evolution of the triplet abundance contains the T decay rate γ D and the T T * annihilation rate γ A :…”

“…(13) agrees with eq. (29c) of [3] (where a factor 2 was however missed in the Boltzmann equation in front of γ A ; the factor 1/24 coming from annihilations into HH * was neglected; the g 4 2 factor was unproperly typed as g 8 2 in the numerical code); ii) eq. (13) differs by order one factors from eq.…”

Leptogenesis beyond the limit of hierarchical heavy neutrino masses

“…There is one possibility, discussed in the case C of Section 4, where the condition on δ 2 to recover the HL becomes more stringent. For quasi-degenerate neutrinos this also provides a way to evade the upper bound on neutrino masses [17].…”