Spurious instabilities in multiangle simulations of collective flavor conversion

TL;DR

This paper investigates the numerical challenges in simulating collective neutrino flavor conversion in supernovae, highlighting how discretization in angle bins can cause unphysical instabilities and proposing a stability analysis to understand these effects.

Contribution

It provides a linearized stability analysis to explain spurious instabilities caused by discretization in multiangle neutrino simulations.

Findings

Small angle bin numbers lead to deep, unphysical flavor conversion.

Single-angle approximation can approximate the infinite-angle limit.

Discretization effects can cause artificial instabilities in simulations.

Abstract

The dense neutrino flux streaming from a core-collapse supernova can undergo self-induced flavor conversion caused by neutrino-neutrino refraction. Numerical studies of these nonlinear effects are challenging because representing the neutrino radiation field by discrete energy and angle bins can easily lead to unphysical solutions. In particular, if the number of angle bins N_a is too small, flavor conversion begins too deep and produces completely spurious results. At the same time, N_a=1 (single-angle approximation) can be a good proxy for the N_a -> infinity limit. Based on a linearized stability analysis, we explain some of the puzzling effects of discrete angle distributions.