Time-Dependent and Quasisteady Features of Fast Neutrino-Flavor Conversion

Despite the theoretical indication that fast neutrino-flavor conversion (FFC) ubiquitously occurs iin corecollapse supernovae and binary neutron star mergers, the lack of global simulations has been the greatest obstacle to study their astrophysical consequences. In this Letter, we present large-scale (50 km) simulations of FFC in spherical symmetry by using a novel approach. We effectively rescale the oscillation scale of FFC by reducing the number of injected neutrinos in the simulation box, and then extrapolate back to the case of the target density of neutrinos with a convergence study. We find that FFC in all models achieves a quasisteady state in the nonlinear regime, and its saturation property of FFC is universal. We also find that temporal- and spatial variations of FFC are smeared out at large radii due to phase cancellation through neutrino self-interactions. Finally, we provide a new diagnostic quantity, electron neutrino lepton number subtracted by heavy one angular crossing, to assess the nonlinear saturation of FFC.