Role of dense matter in collective supernova neutrino transformations

Abstract

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 ${n}_{\ensuremath{\nu}}$ in the conversion region. If ${n}_{e}$ is comparable to ${n}_{\ensuremath{\nu}}$, one finds multiangle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region ${n}_{\ensuremath{\nu}}$ 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.