Collisional flavor swap with neutrino self-interactions

Neutrinos play pivotal roles in determining fluid dynamics, nucleosynthesis, and their observables in core-collapse supernova (CCSN) and binary neutron star merger (BNSM). In this paper, we present a novel phenomenon, collisional flavor swap, in which neutrino-matter interactions trigger the complete interchange of neutrino spectra between two different flavors, aided by neutrino self-interactions. We find that a necessary condition to trigger the collisional swap is occurrences of resonance-like collisional flavor instability. In cases where neutrino self-interactions substantially dominate over the collision rate, the collisional swap occurs in the entire neutrino energy spectrum, while intriguing energy dependent features can emerge after the completion of flavor swap. Since flavor swaps correspond to the most extreme case in flavor conversions, they have a great potential to affect CCSN and BNSM phenomena.