Time-dependent and quasi-steady features of fast neutrino-flavor conversion

TL;DR

This paper presents large-scale spherical simulations of fast neutrino-flavor conversion (FFC), revealing universal saturation properties and the smearing of variations at large radii, advancing understanding of FFC in astrophysical environments.

Contribution

The study introduces a novel rescaling approach for large-scale FFC simulations and identifies universal saturation and phase cancellation effects.

Findings

FFC achieves quasi-steady state in non-linear regime

Saturation property of FFC is universal

Temporal and spatial variations are smeared out at large radii

Abstract

Despite the theoretical indication that fast neutrino-flavor conversion (FFC) ubiquitously occurs in core-collapse supernova and binary neutron star merger, the lack of global simulations has been the greatest obstacle to study their astrophysical consequences. In this {\it Letter}, we present large-scale ($50 {\rm km}$) 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 quasi-steady state in the non-linear 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, ELN-XLN angular crossing, to assess the non-linear saturation of FFC.