Further studies on relic neutrino asymmetry generation II: a rigorous   treatment of repopulation in the adiabatic limit
  We derive an approximate relic neutrino asymmetry evolution equation that systematically incorporates repopulation processes from the full quantum kinetic equations (QKEs). It is shown that in the collision dominant epoch, the said equation reduces precisely to the expression obtained previously from the static/adiabatic approximation. The present treatment thus provides a rigorous justification for the seemingly incongruous assumptions of a negligible repopulation function and instantaneous repopulation sometimes employed in earlier works. 
Keith S. M. Lee, Raymond R. Volkas and Yvonne Y. Y. Wong