Self-induced flavor conversion of supernova neutrinos

Neutrino flavor evolution in dense media such as core-collapse supernovae is dominated by neutrino–neutrino refraction, producing ``self-induced flavor conversion,” i.e., shuffling of flavor among momentum modes. This effect is driven by collective run-away modes of the coupled ``flavor oscillators” and can spontaneously break the initial symmetries (such as axial symmetry, homogeneity, isotropy, and even stationarity) of the neutrino flux and in particular can produce small-scale features. Another interesting feature of these unstable modes are that the growth rates can be of the order of the neutrino–neutrino interaction energy instead of the much smaller vacuum oscillation frequency: implying that the self-induced flavor conversion does not always require neutrino masses. However, the unavoidable ``multi-angle matter effect'' may shift these small-scale instabilities into regions of matter and neutrino density which are not encountered on the way out from a SN. We illustrate these newly found phenomena in terms of simple toy models, what happens in realistic astrophysical scenarios is remains to be understood.