Fast Neutrino-Flavor Conversion with Attenuation and Global Lepton Gradient

Fast neutrino-flavor conversion (FFC) can nontrivially alter neutrino radiation field in core-collapase supernovae (CCSN) and binary neutron-star merger (BNSM) remnants. However, its interplay with global geometry remains poorly understood because microscopic flavor conversion scales are much shorter than global transport scales. We perform global quantum kinetic neutrino transport simulations in spherical geometry with neutrino and matter backgrounds, using an attenuated oscillation Hamiltonian. We find that steep radial lepton gradients can suppress FFC, whereas the suppression is highly sensitive to the adopted attenuation parameter. This behavior is explained by an adiabatic condition: flavor coherence can grow sufficiently only while the flavor wave remains on the unstable branch in the local dispersion relation during propagation. Background variation shifts the unstable branch, while attenuation lengthens the growth timescale, making the flavor coherence following more difficult. We provide an approximate formula for the adiabaticity that can be used directly in CCSN and BNSM models developed by classical neutrino transport simulations. Our results show that attenuation artificially leads to an overestimation of the impact of background variation and should therefore be applied with caution in global simulations of neutrino flavor conversion.