Fast neutrino-flavor conversion in the preshock region of core-collapse supernovae

We make a strong case that the fast neutrino-flavor conversion, one of the collective flavor oscillation modes, commonly occurs in core-collapse supernovae (CCSNs). It is confirmed in the numerical data obtained in realistic simulations of CCSNs, but the argument is much more generic and applicable universally: The coherent neutrino-nucleus scattering makes the electron lepton number (ELN) change sign at some inward direction and trigger the flavor conversion in the outward direction in the preshock region. Although the ELN crossing is tiny and that is why it has eluded recognition so far, it is still large enough to induce the flavor conversion. Our findings may have important observational consequences for CCSN neutrinos.