Role of dense matter in collective supernova neutrino transformations
  For neutrinos streaming from a supernova (SN) core, dense matter suppresses self-induced flavor transformations if the electron density n_e significantly exceeds the neutrino density n_nu in the conversion region. If n_e is comparable to n_nu one finds multi-angle decoherence, whereas the standard self-induced transformation behavior requires that in the transformation region n_nu is safely above n_e. This condition need not be satisfied in the early phase after supernova core bounce. Our new multi-angle effect is a subtle consequence of neutrinos traveling on different trajectories when streaming from a source that is not point-like. 
A. Esteban-Pretel, A. Mirizzi, S. Pastor, R. Tomas, G.G. Raffelt, P.D.   Serpico, G. Sigl