Decoherence in supernova neutrino transformations suppressed by deleptonization

In the dense-neutrino region at 50–400 km above the neutrino sphere in a supernova, neutrino-neutrino interactions cause large flavor transformations. We study when the multiangle nature of the neutrino trajectories leads to flavor decoherence between different angular modes. We consider a two-flavor mixing scenario between nu_e and another flavor nu_x and assume the usual hierarchy F_nue>F_{[overline nu ][sub e]}>F_nux=F_{[overline nu ][sub x]} for the number fluxes. We define epsilon=(F_nue-F_{[overline nu ][sub e]})/(F_{[overline nu ][sub e]}-F_{[overline nu ][sub x]}) as a measure for the deleptonization flux which is the one crucial parameter. The transition between the quasi-single-angle behavior and multiangle decoherence is abrupt as a function of epsilon. For typical choices of other parameters, multiangle decoherence is suppressed for epsilon>~0.3, but a much smaller asymmetry suffices if the neutrino mass hierarchy is normal and the mixing angle small. The critical epsilon depends logarithmically on the neutrino luminosity. In a realistic supernova scenario, the deleptonization flux is probably enough to suppress multiangle decoherence.

©2007 The American Physical Society