Neutrino Fast Flavor Oscillations with Moments: Linear Stability Analysis and Application to Neutron Star Mergers

Providing an accurate modeling of neutrino physics in dense astrophysicalenvironments such as binary neutron star mergers presents a challenge forhydrodynamic simulations. Nevertheless, understanding how flavor transformationcan occur and affect the dynamics, the mass ejection, and the nucleosynthesiswill need to be achieved in the future. Computationally expensive, large-scalesimulations frequently evolve the first classical angular moments of theneutrino distributions. By promoting these quantities to matrices in flavorspace, we develop a linear stability analysis of fast flavor oscillations usingonly the first two "quantum" moments, which notably requires generalizing theclassical closure relations that appropriately truncate the hierarchy of momentequations in order to treat quantum flavor coherence. After showing theefficiency of this method on a well-understood test situation, we perform asystematic search of the occurrence of fast flavor instabilities in a neutronstar merger simulation. We discuss the successes and shortcomings of momentlinear stability analysis, as this framework provides a time-efficient way todesign and study better closure prescriptions in the future.