For neutrinos streaming from a supernova core, dense matter suppresses self-induced flavor transformations if the electron density $n_e$ significantly exceeds the neutrino density $n_{\nu }$ in the conversion region. If $n_e$ is comparable to $n_{\nu }$, one finds multiangle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region $n_{\nu }$ is safely above $n_e$. 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.

• Received 16 July 2008

DOI:https://doi.org/10.1103/PhysRevD.78.085012