Once-in-a-lifetime encounter models for neutrino media: Evolution of flavor equilibria

The once-in-a-lifetime encounter (OILE) models have been used to study collective flavor oscillations in dense neutrino media such as core-collapse supernovae and neutron-star mergers. They keep track of the quantum entanglement among neutrinos via one-particle density operators. We find that a neutrino medium in such a model can evolve along a path of mean-field equilibria, driven adiabatically by the collisional effect due to entanglement. For a homogeneous and isotropic neutrino medium, these mean-field equilibria have been previously well-studied. We find the equilibria for the anisotropic case and show that adiabatic evolution along them matches numerical results. This study may find applications in realistic simulations of astrophysical events such as core-collapse supernovae.