The importance of N2 leptogenesis
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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 $T\gsim 10^9$ 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 $L_p \approx 0$. For $T\lsim 10^9$ GeV, $L_p$ is generally erased, unless special alignment/orthogonality conditions in flavor space are realized.
5 pages. A few clarifications added, conclusions unchanged. Version published in Phys. Rev. Lett. (Title changed in journal)
5 pages. A few clarifications added, conclusions unchanged. Version published in Phys. Rev. Lett. (Title changed in journal)