Neutrino fast flavor oscillations with moments: linear stability analysis and application to neutron star mergers

TL;DR

This paper develops a linear stability analysis method using moments to efficiently study fast neutrino flavor oscillations in dense astrophysical environments like neutron star mergers, aiding future modeling efforts.

Contribution

It introduces a moment-based linear stability analysis framework for neutrino flavor oscillations, generalizing classical closure relations for quantum flavor coherence.

Findings

Efficient detection of flavor instabilities in neutron star merger simulations

Identification of limitations in current moment closure approaches

Framework aids in designing better closure prescriptions for neutrino modeling

Abstract

Providing an accurate modeling of neutrino physics in dense astrophysical environments such as binary neutron star mergers presents a challenge for hydrodynamic simulations. Nevertheless, understanding how flavor transformation can occur and affect the dynamics, the mass ejection, and the nucleosynthesis will need to be achieved in the future. Computationally expensive, large-scale simulations frequently evolve the first classical angular moments of the neutrino distributions. By promoting these quantities to matrices in flavor space, we develop a linear stability analysis of fast flavor oscillations using only the first two "quantum" moments, which notably requires generalizing the classical closure relations that appropriately truncate the hierarchy of moment equations in order to treat quantum flavor coherence. After showing the efficiency of this method on a well-understood test situation, we perform a systematic search of the occurrence of fast flavor instabilities in a neutron star merger simulation. We discuss the successes and shortcomings of moment linear stability analysis, as this framework provides a time-efficient way to design and study better closure prescriptions in the future.