Quantum Zeno effect and the impact of flavour 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 flavour-sensitive and the coherence of their flavour 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 flavour-sensitive processes are fast on the cosmic expansion timescale, they must be fast relative to the N -> l + F reactions lest the flavour amplitudes of l remain frozen by the repeated N -> l + Phi 'measurements'. Our more restrictive requirement is significant in the most interesting 'strong washout case' where N -> l + Phi is fast relative to the cosmic expansion rate. We derive conditions for the unflavoured treatment to be adequate and for flavour effects to be maximal. In this 'fully flavoured 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.