Fast neutrino-flavor swap in high-energy astrophysical environments

We assert that nonlinear features of fast neutrino-flavor conversion (FFC) can be qualitatively different between core-collapse supernovae (CCSNe) and binary neutron star mergers (BNSMs). This argument arises from recent global FFC simulations in BNSM, in which fast flavor swap (FFS) emerges in very narrow spatial regions, whereas neutrinos in CCSN tend to evolve toward flavor equipartition. In this paper, we provide the physical mechanism of FFS based on a colliding neutrino beam model. Neutrinos/antineutrinos can undergo FFS when they propagate in ambient neutrino gas that propagates in the opposite direction and also has the opposite sign of ELN-XLN, where ELN and XLN denote electron- and heavy-leptonic neutrino number, respectively. Such environments can be naturally realized in BNSMs, whereas they are unlikely in CCSNe unless the neutrino sphere is strongly deformed aspherically. Our study exhibits the diversity of nonlinear dynamics of FFC.