Recent developments in thermal leptogenesis: the role of flavours in various seesaw realisations

In this thesis, we study the role of flavours in leptogenesis. In a first time, we review the so-called one flavour picture, where only the lightest right-handed neutrino is taken into account, and where lepton flavours are neglected. After having showed why and how lepton flavours should be included in the computation of the lepton asymmetry, we study their effect in the type I seesaw model, as well as in the type I+II seesaw model, in a specific SO(10) unification realisation. In this model, besides flavours effects, we show that the inclusion of the second lightest right-handed neutrino, together with an accurate description of charged fermions masses, are necessary to make leptogenesis work.