When First Beats Fast: Early Neutrino-Mass-Driven Flavor Instabilities in Supernovae

Collective neutrino flavor conversions in core-collapse supernovae (SNe) begin with instabilities, initially triggered when the dominant νₑ outflow concurs with a small flux of antineutrinos with the opposite lepton number, with νₑ dominating over ν_μ. When these "flipped" neutrinos emerge in the energy-integrated angular distribution (angular crossing), they initiate a fast instability. However, before such conditions arise, spectral crossings typically appear within 20~ms of collapse, i.e., local spectral excesses of νₑ over ν_μ along some direction. Therefore, post-processing SN simulations cannot consistently capture later fast instabilities because the early slow ones have already altered the conditions.