Self-induced flavor conversion of supernova neutrinos on small scales

TL;DR

This paper investigates the conditions under which small-scale flavor conversion instabilities occur in supernova neutrinos, emphasizing the impact of matter effects and the stability of large-scale modes.

Contribution

It demonstrates that small-scale instabilities are suppressed by matter effects, and clarifies the relation between time evolution and radial profiles of supernova neutrino fluxes.

Findings

Small-scale flavor instabilities are shifted out of regions with high matter density.

Large-scale uniform modes are most susceptible to flavor conversion.

The study clarifies the connection between neutrino gas evolution and supernova neutrino flux profiles.

Abstract

Self-induced flavor conversion of supernova (SN) neutrinos is a generic feature of neutrino-neutrino dispersion. The corresponding run-away modes in flavor space can spontaneously break the original symmetries of the neutrino flux and in particular can spontaneously produce small-scale features as shown in recent schematic studies. However, the unavoidable "multi-angle matter effect" shifts these small-scale instabilities into regions of matter and neutrino density which are not encountered on the way out from a SN. The traditional modes which are uniform on the largest scales are most prone for instabilities and thus provide the most sensitive test for the appearance of self-induced flavor conversion. As a by-product we clarify the relation between the time evolution of an expanding neutrino gas and the radial evolution of a stationary SN neutrino flux. Our results depend on several simplifying assumptions, notably stationarity of the solution, the absence of a "backward" neutrino flux caused by residual scattering, and global spherical symmetry of emission.