On the occurrence of fast neutrino flavor conversions in multidimensional supernova models

The dense neutrino medium in a core-collapse supernova or a neutron-star merger event can experience fast flavor conversions on time/distance scales that are much smaller than those of vacuum oscillations. It is believed that fast neutrino flavor transformation occurs in the region where the angular distributions of $\nu_e$ and $\bar\nu_e$ cross each other. We present the first study of this crossing phenomenon and the fast neutrino flavor conversions in multidimensional (multi-D) supernova models. We examine the neutrino distributions obtained by solving the Boltzmann transport equation for several fixed profiles which are representative snapshots taken from separate 2D and 3D supernova simulations with an $11.2 M_\odot$ progenitor model. Our research shows that the spherically asymmetric patterns of the $\nu_e$ and $\bar\nu_e$ fluxes in multi-D models can assist the appearance of the crossing between the $\nu_e$ and $\bar\nu_e$ angular distributions. In the models that we have studied, there exist unstable neutrino oscillation modes in and beyond the neutrino decoupling region which have amplitude growth rates as large as an $e$-fold per nanosecond. This finding can have important consequences for the explosion mechanism, nucleosynthesis, and neutrino signals of core-collapse supernovae.