The Neutrino Fast Flavor Instability in Three Dimensions

TL;DR

This paper investigates the neutrino fast flavor instability in three dimensions, testing the validity of previous symmetry assumptions and assessing the robustness of lower-dimensional simulation results.

Contribution

The study performs comprehensive 3D simulations of neutrino flavor instability, revealing conditions under which symmetry assumptions may suppress or not affect the instability's late-time behavior.

Findings

Fast flavor instability can be suppressed if the fastest growing mode aligns with imposed symmetry directions.

Late-time flavor distributions are largely unaffected by the number of spatial dimensions.

Lower-dimensional simulations may reliably predict certain aspects of neutrino flavor evolution.

Abstract

Neutrino flavor instabilities have the potential to shuffle neutrinos between electron, mu, and tau flavor states, modifying the core-collapse supernova mechanism and the heavy elements ejected from neutron star mergers. Analytic methods indicate the presence of so-called fast flavor transformation instabilities, and numerical simulations can be used to probe the nonlinear evolution of the neutrinos. Simulations of the fast flavor instability to date have been performed assuming imposed symmetries. We perform simulations of the fast flavor instability that include all three spatial dimensions and all relevant momentum dimensions in order to probe the validity of these approximations. If the fastest growing mode has a wavenumber along a direction of imposed symmetry, the instability can be suppressed. The late-time equilibrium distribution of flavor, however, seems to be little affected by the number of spatial dimensions. This is a promising hint that the results of lower-dimensionality simulations to date have predictions that are robust against their the number of spatial dimensions, though simulations of a wider variety of neutrino distributions need to be carried out to support this claim more generally.