Quantum Zeno effect and the impact of flavor in leptogenesis

In thermal leptogenesis, the cosmic matter-antimatter asymmetry is produced by CP violation in the decays N --> l + \Phi of heavy right-handed Majorana neutrinos N into ordinary leptons l and Higgs particles \Phi. If some charged-lepton Yukawa couplings are in equilibrium during the leptogenesis epoch, the l interactions with the background medium are flavor sensitive and the coherence of their flavor content defined by N --> l+\Phi is destroyed, modifying the efficiency of the inverse decays. We point out, however, that it is not enough that the flavor-sensitive processes are fast on the cosmic expansion time scale, they must be fast relative to the N l +\Phi reactions lest the flavor amplitudes of l remain frozen by the repeated N l+\Phi ``measurements''. Our more restrictive requirement is significant in the most interesting ``strong wash-out case'' where N l +\Phi is fast relative to the cosmic expansion rate. We derive conditions for the unflavored treatment to be adequate and for flavor effects to be maximal. In this ``fully flavored regime'' a neutrino mass bound survives. To decide if this bound can be circumvented in the intermediate case, a full quantum kinetic treatment is required.