Elaborating the Ultimate Fate of Fast Collective Neutrino Flavor Oscillations

TL;DR

This paper investigates the ultimate fate of fast collective neutrino flavor oscillations, showing how they lead to depolarization and flavor mixing, with implications for supernova physics.

Contribution

It provides an analytical estimate for the depolarization extent and introduces recipes for depolarized distributions applicable to supernova models.

Findings

Depolarization depends on initial momentum distributions and lepton asymmetry.

Relaxation mechanisms like transverse relaxation and flavor-wave mixing drive flavor equilibration.

Analytical recipes for flavor distributions aid supernova neutrino modeling.

Abstract

Dense clouds of neutrinos and antineutrinos can exhibit fast collective flavor oscillations. Previously, in Phys. Rev. Lett. 126 (2021) 061302, we proposed that such flavor oscillations lead to depolarization, i.e., an irreversible mixing of the flavors, whose extent depends on the initial momentum distributions of the different flavors. In this paper, we elaborate and extend this proposal, and compare it with related results in the literature. We present an accurate analytical estimate for the lower resting point of the fast flavor pendulum and underline the relaxation mechanisms, i.e., transverse relaxation, multipole cascade, and mixing of flavor-waves, that cause it to settle down. We estimate the extent of depolarization, its dependence on momentum and net lepton asymmetry, and its generalization to three flavors. Finally, we prescribe approximate analytical recipes for the depolarized distributions and fluxes that can be used in supernova/nucleosynthesis simulations and supernova neutrino phenomenology.