Multi-azimuthal-angle instability for different supernova neutrino fluxes

TL;DR

This paper investigates how the newly identified multi-azimuthal-angle (MAA) instability influences supernova neutrino flavor conversions, depending on initial flux spectra and spectral crossing patterns, through stability analysis and numerical simulations.

Contribution

It provides a detailed analysis of MAA instability dependence on supernova neutrino fluxes and spectral crossings, highlighting conditions for flavor conversion onset in different hierarchies.

Findings

MAA instability triggers flavor conversions for single-crossing spectra in normal hierarchy.

Multiple spectral crossings delay flavor conversions due to MZA effects.

Spectral features determine the impact of MAA effects on flavor evolution.

Abstract

It has been recently discovered that removing the axial symmetry in the "multi-angle effects" associated with the neutrino-neutrino interactions for supernova (SN) neutrinos, a new multi-azimuthal-angle (MAA) instability would trigger flavor conversions in addition to the ones caused by the bimodal and multi-zenith-angle (MZA) instabilities. We investigate the dependence of the MAA instability on the original SN neutrino fluxes, performing a stability analysis of the linearized neutrino equations of motion. We compare these results with the numerical evolution of the SN neutrino non-linear equations, looking at a local solution along a specific line of sight, under the assumption that the transverse variations of the global solution are small. We also assume that self-induced conversions are not suppressed by large matter effects. We show that the pattern of the spectral crossings (energies where F_{\nu_e} = F_{\nu_x}, and F_{\bar\nu_e} = F_{\bar\nu_x}) is crucial in determining the impact of MAA effects on the flavor evolution. For neutrino spectra with a strong excess of \nu_e over \bar\nu_e, presenting only a single-crossing, MAA instabilities would trigger new flavor conversions in normal mass hierarchy. In our simplified flavor evolution scheme, these would lead to spectral swaps and splits analogous to what produced in inverted hierarchy by the bimodal instability. Conversely, in the presence of spectra with a moderate flavor hierarchy, having multiple crossing energies, MZA effects would produce a sizable delay in the onset of the flavor conversions, inhibiting the growth of the MAA instability. In this case the splitting features for the oscillated spectra in both the mass hierarchies are the ones induced by the only bimodal and MZA effects.