Leptogenesis beyond the limit of hierarchical heavy neutrino masses

We calculate the baryon asymmetry of the Universe in thermal leptogenesis beyond the usual lightest right-handed (RH) neutrino dominated scenario (N1DS) and in particular beyond the hierarchical limit (HL), M1 ≪ M2 ≪ M3, for the RH neutrino mass spectrum. After providing some orientation among the large variety of models, we first revisit the central role of the N1DS, with new insights on the dynamics of the asymmetry generation and then discuss the main routes departing from it, focusing on models beyond the HL. We study in detail two examples of ‘strong-strong’ washout scenarios: one with ‘maximal phase’ and the limit of very large M3, studying the effects arising when δ2 ≡ (M2 − M1)/M1 is small. We extend analytical methods already applied to the N1DS showing, for example, that, in the degenerate limit (DL), the efficiency factors of the RH neutrinos become equal with the single decay parameter replaced by the sum. Both cases disprove the misconception that close RH neutrino masses necessarily lead to a final asymmetry enhancement and to a relaxation of the lower bounds onM1 and on the initial temperature of the radiationdominated expansion. We also explain why leptogenesis tends to favor normal hierarchy compared to inverted hierarchy for the left-handed neutrino masses.