Role of dense matter in collective supernova neutrino transformations

For neutrinos streaming from a supernova core, dense matter suppresses self-induced flavor transformations if the electron density nₑ significantly exceeds the neutrino density n_ν in the conversion region. If nₑ is comparable to n_ν, one finds multiangle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region n_ν is safely above nₑ. This condition need not be satisfied in the early phase after the supernova core bounce. Our new multiangle effect is a subtle consequence of neutrinos traveling on different trajectories when streaming from a source that is not pointlike.