Emergence of coherent flavor oscillations in dense neutrino gases: The once-in-a-lifetime model

The mean-field approximation is the standard approach to collective neutrino oscillations in astrophysical contexts, but its validity remains debated. We introduce a toy model for flavor oscillations that transcends the mean-field paradigm by incorporating many-body effects, such as entanglement between neutrinos, in a way that captures the once-in-a-lifetime encounter (OILE) idea: a neutrino typically encounters any other particular neutrino at most once its lifetime. From our toy model, coherent flavor oscillations, flavor decoherence, and flavor equilibration emerge. We present numerical simulations of flavor dynamics in the model accompanied by matching analytic results.