Analysis of matter suppression in collective neutrino oscillations during the supernova accretion phase

The usual description of self-induced neutrino flavor conversions in core-collapse supernovae is based on the dominance of the neutrino density n(v) over the net electron density n(e). However, this condition is not met during the post-bounce accretion phase, when the dense matter in a SN is piled up above the neutrinosphere. As recently pointed out, a dominant matter term in the anisotropic SN environment would dephase the flavor evolution for neutrinos traveling on different trajectories, challenging the occurrence of the collective behavior in the dense neutrino gas. Using the results from recent long-term simulations of core-collapse SN explosions, based on three-flavor Boltzmann neutrino transport in spherical symmetry, we find that both the situations of complete matter suppression (when n(e) >> n(v)) and matter-induced decoherence (when n(e) greater than or similar to n(v)) of flavor conversions are realized during the accretion phase. The matter suppression at high densities prevents any possible impact of the neutrino oscillations on the neutrino heating and hence on the dynamics of the explosion. Furthermore, it changes the interpretation of the Earth matter effect on the SN neutrino signal during the accretion phase, allowing the possibility of the neutrino mass hierarchy discrimination at not too small values of the leptonic mixing angle theta(13) (i.e., sin(2)theta(13) greater than or similar to 10(-3)).