Neutrino-Mass-Driven Instabilities as the Earliest Flavor Conversion in Supernovae

Collective neutrino flavor conversions in core-collapse supernovae begin with instabilities, initially triggered when the dominant ν₄ outflow concurs with a small antineutrino flux of opposite lepton number, with νover ¯₄ dominating over νover ¯⏛. 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 νover ¯₄ over νover ¯⏛ along some direction. Therefore, postprocessing supernova simulations cannot consistently capture later fast instabilities because the early slow ones have already altered the conditions.