Flavor instabilities in the multi-angle neutrino line model

TL;DR

This paper investigates neutrino flavor oscillation instabilities in a multi-angle line model, revealing that inhomogeneous modes can become unstable at higher densities and are sensitive to matter effects, complicating numerical simulations.

Contribution

The study extends previous two-beam analyses by developing a numerical code for the multi-angle model and demonstrating the conditions for inhomogeneous mode instabilities at various densities.

Findings

Inhomogeneous modes occur at higher densities in the multi-angle model.

Small-scale inhomogeneous modes can be unstable at lower densities with matter.

Numerical treatment of these modes is challenging due to spurious oscillations.

Abstract

Neutrino flavor oscillations in the presence of ambient neutrinos is nonlinear in nature which leads to interesting phenomenology that has not been well understood. It was recently shown that, in the two-dimensional, two-beam neutrino Line model, the inhomogeneous neutrino oscillation modes on small distance scales can become unstable at larger neutrino densities than the homogeneous mode does. We develop a numerical code to solve neutrino oscillations in the multi-angle/beam Line model with a continuous neutrino angular distribution. We show that the inhomogeneous oscillation modes can occur at even higher neutrino densities in the multi-angle model than in the two-beam model. We also find that the inhomogeneous modes on sufficiently small scales can be unstable at smaller neutrino densities with ambient matter than without, although a larger matter density does shift the instability region of the homogeneous mode to higher neutrino densities in the Line model as it does in the one-dimensional supernova Bulb model. Our results suggest that the inhomogeneous neutrino oscillation modes can be difficult to treat numerically because the problem of spurious oscillations becomes more severe for oscillations on smaller scales.