We examine a phenomenon recently predicted by numerical simulations of supernova neutrino flavor evolution: the swapping of supernova νe and νμ,τ energy spectra below (above) energy EC for the normal (inverted) neutrino mass hierarchy. We present the results of large-scale numerical calculations which show that in the normal neutrino mass hierarchy case, EC decreases as the assumed effective 2×2 vacuum νe νμ,τ mixing angle (θ13) is decreased. In contrast, these calculations indicate that EC is essentially independent of the vacuum mixing angle in the inverted neutrino mass hierarchy case. With a good neutrino signal from a future galactic supernova, the above results could be used to determine the neutrino mass hierarchy even if θ13 is too small to be measured by terrestrial neutrino oscillation experiments.
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