Collective neutrino oscillations in supernovae: Matter suppression during the accretion phase

We carry out a detailed analysis of the supernova (SN) neutrino flavor evolution during the accretion phase (at post-bounce times t(pb) <= 500 ms), characterizing the SN nu signal by recent hydrodynamical simulations. We find that trajectory-dependent "multi-angle" effects, associated with the dense ordinary matter suppress collective oscillations, that would have been induced by nu - nu interactions in the deepest SN regions. The matter suppression implies that neutrino oscillations will start outside the neutrino decoupling 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 likely case that the mixing angle Theta(13) is not very small