Importance of the heavier singlet neutrinos in leptogenesis

We argue that fast interactions of the lightest singlet neutrino N1 would project part of a preexisting lepton asymmetry Lp onto a direction that is protected from N1 washout effects, thus preventing it from being erased. In particular, we consider an asymmetry generated in N2 decays, assuming that N1 interactions are fast enough to bring N1 into full thermal equilibrium. If N1 decays occur at T 109GeV, that is, before the muon Yukawa interactions enter into thermal equilibrium, then generically part of Lp survives. In this case some of the constraints implied by the standard N1 leptogenesis scenario hold only if Lp≈0. For T 109GeV, Lp is generally erased, unless special alignment or orthogonality conditions in flavor space are realized.