The problem of the initial conditions in flavoured leptogenesis and the tauon N2-dominated scenario

We discuss the conditions to realise a scenario of 'strong thermal leptogenesis,' where the final asymmetry is fully independent of the initial conditions, taking into account both heavy and light neutrino flavour effects. In particular, the contribution to the final asymmetry from an 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 an 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 >> 10(12) GeV (heavy flavoured scenario), a result that can be extended to any other mass pattern. (C) 2011 Elsevier B.V. All rights reserved.