Neutrino Quantum Kinetics with Moments in Neutron Star Mergers*

Core-collapse supernovae and neutron star mergers produce multimessenger transient signals that clue us in to the origin of the elements in the universe, the behavior of dense matter, and the nature of fundamental particles. Neutrinos of different flavors in these systems carry energy and lepton number, modifying the light, neutrino, and nucleosynthetic signature we see. Properly modeling this flavor transformation is extremely difficult, since flavor changing physics occurs on length scales of as small as millimeters, while the explosive dynamics occur on more than hundreds or kilometers. We simulate neutrino flavor transformation in a 3D cube extracted from a neutron star merger with two methods: an exact particle-in-cell simulation and an approximate two-moment simulation. We show that the resulting flavor instability is similar in both cases and reinforce the results with an analytic description of flavor instability in both cases. The ability of a moment method to exhibit such similar flavor instability suggests that self-consistent flavor transformation physics could become a standard component of future supernova and merger simulations.