Predicting the outcome of collisional neutrino flavor conversion

Abstract: Collisional flavor instabilities, driven by differing neutrino and antineutrino reaction rates, are expected to occur in dense astrophysical environments like supernovae and neutron star mergers, but have yet to be incorporated in large-scale simulations. We derive analytical expressions for the asymptotic state resulting from a homogeneous and isotropic instability, and apply these predictions to two representative conditions from a neutron star merger simulation. We emphasize the importance of using a collision term that allows for both damping of flavor coherence and relaxation back to the classical steady-state. When this classical configuration is collisional-unstable, the resulting asymptotic state reflects a compromise between classical relaxation and flavor conversion, defining a "quantum" equilibrium with nonzero coherence. This analysis highlights the possibility of a trade-off between classical and quantum effects, an important feature with regard to the inclusion of flavor oscillation physics into global simulations.