Flavor decoherence and washout timing in nonthermal leptogenesis after inflation

We investigate why changing the heavy-neutrino mass hierarchy can reverse the baryon asymmetry in nonthermal leptogenesis after hilltop inflation. Along the mass trajectory studied here, lowering the mass of the lighter state, $N_1$, reverses the final asymmetry while leaving the dominant injection from $N_2$ decays nearly unchanged. The mass change primarily delays $N_1$ washout relative to electron–muon flavor decoherence; at fixed source and background, translating the washout history while preserving its shape and integrated strength reproduces more than 99% of the yield changes from the nonthermal $N_2$ source at two physical target masses. Decomposing the $N_2$ CP source shows that the projector-aligned component remains positive under both washout histories, whereas the initially trace-free component reverses its propagated contribution and accounts for about 99.7% of the timing contrast in the fixed-source comparison. Rapid damping of $e\mu$ flavor coherence largely removes this contrast, while rapid chemical equilibration does not. Later washout therefore acts on a more decohered flavor state, reducing the positive $e\mu$ contribution and allowing negative diagonal contributions to determine the final sign. At a mass recomputed to reproduce the observed baryon yield, a 10% increase of the muon collision operator reverses the sign in a fixed-input sensitivity test.