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 n₄ significantly exceeds the neutrino density n_ in the conversion region. If n₄ is comparable to n_, one finds multiangle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region n_ is safely above n₄. 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.