In the environment with dense neutrino gases such as in core-collapse supernovae (CCSNe), the neutrino self-interactions are potentially dominant, and collective flavor conversions occur near the core. In particular, the crossings in the neutrino angular distribution can induce fast flavor conversion. The coherent scatterings off heavy nuclei in the preshock region of CCSNe can create tiny negative crossings in the backward direction and lead to fast instability. We demonstrate the flavor evolution triggered by the fast instability and a cascade due to the nonlinear effects.
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