No collective neutrino flavor conversions during the supernova accretion phase

The large neutrino fluxes emitted with a distinct flavor hierarchy from core-collapse supernovae (SNe) during the post-bounce accretion phase, offer the best opportunity to detect effects from neutrino flavor oscillations. We perform a dedicated study of the SN neutrino flavor evolution during the accretion phase, using results from recent neutrino radiation hydrodynamics simulations. In contrast to what expected in the presence of only neutrino-neutrino interactions, we find that the multi-angle effects associated with the dense ordinary matter suppress collective oscillations. This is related to the high matter densities during the accretion phase in core-collapse SNe of massive iron-core progenitors. The matter suppression implies that neutrino oscillations will start outside the neutrino transport region and therefore will have a negligible impact on the neutrino heating and the explosion dynamics. Furthermore, the possible detection of the next galactic SN neutrino signal from the accretion phase, based on the usual Mikheyev- Smirnov-Wolfenstein effect in the SN mantle and Earth matter effects, can reveal the neutrino mass hierarchy in the case that the mixing angle $\theta_{13}$ is not very small.