Neutrino Flavor Transformation in Supernovae

Neutrino flavor transformation can be very important for both the explosion and nucleosynthesis in a core-collapse supernova event. We have studied the coherent neutrino flavor transformation outside the supernova core a few seconds after the bounce. We found that neutrino flavor transformation in supernovae can possess collective features which are absent in the solar neutrino mixing problem. These collective features can be understood with simple analytical models and are supported in our recent large-scale numerical simulations. Neutrino mass hierarchy information might be inferred from the analyses of future supernova neutrino signals.