Importance of the heavier singlet neutrinos in leptogenesis

We argue that fast interactions of the lightest singlet neutrino N-1 would project part of a preexisting lepton asymmetry L-p onto a direction that is protected from N-1 washout effects, thus preventing it from being erased. In particular, we consider an asymmetry generated in N-2 decays, assuming that N-1 interactions are fast enough to bring N-1 into full thermal equilibrium. If N-1 decays occur at T greater than or similar to 10(9) GeV, that is, before the muon Yukawa interactions enter into thermal equilibrium, then generically part of L-p survives. In this case some of the constraints implied by the standard N-1 leptogenesis scenario hold only if L-p approximate to 0. For T less than or similar to 10(9) GeV, L-p is generally erased, unless special alignment or orthogonality conditions in flavor space are realized.