Neutrino Flavor Conversion, Advection, and Collisions: Towards the Full Solution

TL;DR

This paper presents the first comprehensive simulations of neutrino flavor evolution in dense astrophysical environments, incorporating flavor conversion, collisions, and advection, revealing complex interplay and simultaneous slow and fast transformations.

Contribution

It introduces the first spherical symmetry simulations that include neutrino-neutrino scattering, collisions, and advection, challenging previous assumptions about flavor equipartition.

Findings

Flavor equipartition is not a universal outcome.

Flavor conversion spreads across angular distributions and regions.

Slow and fast flavor transformations can occur simultaneously.

Abstract

At high densities in compact astrophysical sources, the coherent forward scattering of neutrinos onto each other is responsible for making the flavor evolution non-linear. Under the assumption of spherical symmetry, we present the first simulations tracking flavor transformation in the presence of neutrino-neutrino forward scattering, neutral and charged current collisions with the matter background, as well as neutrino advection. We find that, although flavor equipartition could be one of the solutions, it is not a generic outcome, as often postulated in the literature. Intriguingly, the strong interplay between flavor conversion, collisions, and advection leads to a spread of flavor conversion across the neutrino angular distributions and neighboring spatial regions. Our simulations show that slow and fast flavor transformation can occur simultaneously. In the light of this, looking for crossings in the electron neutrino lepton number as a diagnostic tool of the occurrence of flavor transformation in the high-density regime is a limiting method.