The problem of the initial conditions in flavoured leptogenesis and the tauon N_2-dominated scenario

TL;DR

This paper analyzes conditions for strong thermal leptogenesis, emphasizing the tauon N_2-dominated scenario with hierarchical right-handed neutrinos, and explores its natural realization in SO(10)-inspired models, including effects of heavy neutrino flavors.

Contribution

It identifies specific hierarchical conditions and flavor effects necessary for successful strong thermal leptogenesis, especially highlighting the tauon N_2-dominated scenario within SO(10)-inspired models.

Findings

Successful strong thermal leptogenesis requires tauon N_2 dominance.

Hierarchical right-handed neutrino masses are essential.

The scenario is naturally realized in SO(10)-inspired models.

Abstract

We discuss the conditions to realize a scenario of strong thermal leptogenesis, where the final asymmetry is fully independent of the initial conditions, when both heavy and light flavor effects are taken into account. In particular the contribution to the final asymmetry from a residual initial pre-existing asymmetry has to be negligible. We show that in the case of a hierarchical right-handed (RH) neutrino mass spectrum, the only possible way is a N_2-dominated leptogenesis scenario with a lightest RH neutrino mass M_1 << 10^9 GeV and with a next-to-lightest RH neutrino mass 10^12 GeV >> M_2 >> 10^9 GeV. This scenario necessarily requires the presence of a heaviest third RH neutrino specie. Moreover, we show that the final asymmetry has to be dominantly produced in the tauon flavour while the electron and the muon asymmetries have to be efficiently washed out by the lightest RH neutrino inverse processes. Intriguingly, such seemingly special conditions for successful strong thermal leptogenesis are naturally fulfilled within SO(10)-inspired models. Besides the tauon N_2-dominated scenario, successful strong thermal leptogenesis is also achieved in scenarios with quasi-degenerate RH neutrino masses. We also comment on the supersymmetric case. We also derive an expression for the final asymmetry produced from leptogenesis taking fully into account heavy neutrino flavour effects in the specific case $M_1\gg 10^{12}\,{\rm GeV}$ (heavy flavoured scenario), a result that can be extended to any other mass pattern.