Successful N₂ leptogenesis with flavour coupling effects in realistic unified models

In realistic unified models involving so-called SO(10)-inspired patterns of Dirac and heavy right-handed (RH) neutrino masses, the lightest right-handed neutrino N₁ is too light to yield successful thermal leptogenesis, barring highly fine tuned solutions, while the second heaviest right-handed neutrino N₂ is typically in the correct mass range. We show that flavour coupling effects in the Boltzmann equations may be crucial to the success of such N₂ dominated leptogenesis, by helping to ensure that the flavour asymmetries produced at the N₂ scale survive N₁ washout. To illustrate these effects we focus on N₂ dominated leptogenesis in an existing model, the A to Z of flavour with Pati-Salam, where the neutrino Dirac mass matrix may be equal to an up-type quark mass matrix and has a particular constrained structure. The numerical results, supported by analytical insight, show that in order to achieve successful N₂ leptogenesis, consistent with neutrino phenomenology, requires a "flavour swap scenario" together with a less hierarchical pattern of RH neutrino masses than naively expected, at the expense of some mild fine-tuning. In the considered A to Z model neutrino masses are predicted to be normal ordered, with an atmospheric neutrino mixing angle well into the second octant and the Dirac phase δ≃20^∘, a set of predictions that will be tested in the next years in neutrino oscillation experiments. Flavour coupling effects may be relevant for other SO(10)-inspired unified models where N₂ leptogenesis is necessary