Successful $N_2$ leptogenesis with flavour coupling effects in realistic unified models

TL;DR

This paper demonstrates that flavour coupling effects can enable successful $N_2$ leptogenesis in realistic $SO(10)$-like models, even when the lightest right-handed neutrino is too light for thermal leptogenesis.

Contribution

It shows how flavour coupling effects in Boltzmann equations can facilitate $N_2$ dominated leptogenesis in unified models, requiring a flavour swap scenario and less hierarchical neutrino masses.

Findings

Flavour coupling effects are crucial for $N_2$ leptogenesis success.

A less hierarchical RH neutrino mass pattern is needed.

Successful leptogenesis aligns with neutrino phenomenology.

Abstract

In realistic unified models involving an $SO(10)$-like pattern of Dirac and heavy right-handed (RH) neutrino masses, the lightest right-handed neutrino $N_1$ is too light to yield successful thermal leptogenesis, barring highly fine tuned solutions, while the second heaviest right-handed neutrino $N_2$ 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 $N_2$ dominated leptogenesis, by helping to ensure that the flavour asymmetries produced at the $N_2$ scale survive $N_1$ washout. To illustrate these effects we focus on $N_2$ 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 $N_2$ 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. These results may be relevant for other $SO(10)$-like unified models where $N_2$ leptogenesis is necessary.