Searching for Fast Neutrino Flavor Conversion Modes in Core-collapse Supernova Simulations

Neutrinos propagating in dense neutrino media such as those in core-collapse supernovae can experience fast flavor conversions on scales much shorter than those expected in vacuum. It is believed that a necessary condition for the occurrence of fast modes is that the angular distributions of $\nu_e$ and $\bar\nu_e$ cross each other. However, most of the state-of-the-art supernova simulations do not provide such detailed angular information and instead, consider only a few moments of neutrino angular distributions. We here propose an efficient method to use these available few moments to search for fast modes in supernova simulations. Our method, which is based on searching for crossings in the angular distributions, can work with any number of moments provided by the simulation though a larger number of crossings can be captured when higher moments are available.