Fast flavor instabilities and the search for neutrino angular crossings

TL;DR

This paper investigates methods to identify neutrino angular crossings related to fast flavor instabilities in supernovae, highlighting limitations of current simulation approaches and comparing reconstruction techniques for better detection accuracy.

Contribution

It critically examines existing methods for detecting angular crossings in neutrino simulations and compares two reconstruction approaches to improve the identification of flavor instabilities.

Findings

Standard closure prescriptions may miss real crossings.

Some closures introduce false crossings.

Reconstruction methods vary in accuracy.

Abstract

With the recognition that fast flavor instabilities likely affect supernova and neutron-star-merger neutrinos, using simulation data to pin down when and where the instabilities occur has become a high priority. The effort faces an interesting problem. Fast instabilities are related to neutrino angular crossings, but simulations often employ moment methods, sacrificing momentum-space angular resolution in order to allocate resources elsewhere. How can limited angular information be used most productively? The main aims here are to sharpen this question and examine some of the available answers. A recently proposed method of searching for angular crossings is scrutinized, the limitations of moment closures are highlighted, and two ways of reconstructing angular distributions solely from the flux factors (based respectively on maximum-entropy and sharp-decoupling assumptions) are compared. In (semi)transparent regions, the standard closure prescriptions likely miss some crossings that should be there and introduce others that should not.