NEUTRINO FLAVOR EVOLUTION NEAR A SUPERNOVAS CORE

In supernovae and in the early universe, neutrino flavor evolution is a many-body phenomena. Here the equations describing the evolution of the density matrices in phase space are derived. Then these equations are applied to neutrino emission from a supernova core. The effects of a ''small'' background neutrino density on adiabatic and nonadiabatic flavor evolution are calculated analytically. It is found that when f lavor evolution is sizeable, the sensitivity to the small neutrino bac kground is enhancer. This implies that r-process nucleosynthesis in su pernovae may not reliably probe neutrino masses less than about 25 eV.