Rates of neutrino conversion and decay in hot and dense QED plasma

Using a real-time formalism of equilibrium and nonequilibrium quantum-field theory, we derive the reaction-rate formula for the neutrino-conversion (ν→ν') process and νν-bar' annihilation process, which take place in a hot and dense QED plasma with background (anti)neutrinos out of equilibrium. Also derived is the formula for the inverse processes to the above ones. Using the hard-thermal-loop resummation scheme, we include the contribution from the coherent processes. The decay or production of a neutrino causes an evolution of its spatial distribution. A scheme for dealing with this evolution is presented. For the case of the isotropic neutrino distribution, a numerical computation is carried out for the parameter region of a type-II supernova explosion. The differential reaction rate exhibits a characteristic peak structure, which comes from the coherent processes. The contribution from the above processes to the decay or damping rate of a parent neutrino ν is also studied