Relic neutrino asymmetry evolution from first principles

The exact quantum kinetic equations for a two-flavor active-sterile neutrino system are used to provide a systematic derivation of approximate evolution equations for the relic neutrino asymmetry. An extension of the adiabatic approximation for matter-affected neutrino oscillations is developed which incorporates decoherence due to collisions. Exact and approximate expressions for the decoherence and repopulation functions are discussed. A first pass is made at an exact treatment of multi-flavor partially incoherent oscillations.