Comment on "Neutrino oscillations in the early universe: How can large lepton asymmetry be generated?" [Astropart. Phys. 14 (2000) 79–90]

Abstract

We comment on the recent paper by A.D. Dolgov, S.H. Hansen, S. Pastor and D.V. Semikoz (DHPS) [Astropart. Phys. 14 (2000) 79] on the generation of neutrino asymmetries from active-sterile neutrino oscillations. We demonstrate that the approximate asymmetry evolution equation obtained therein is an expansion, up to a minor discrepancy, of the well-established static approximation equation, valid only when the supposedly new higher order correction term is small. In the regime where this so-called "back-reaction" term is large and artificially terminates the asymmetry growth, their evolution equation ceases to be a faithful approximation to the quantum kinetic equations simply because pure Mik..