The freeze-out of massless particles is investigated. The effects due to
quantum statistics, Fermi-Dirac or Bose-Einstein, of all particles
relevant for the process are analyzed. Solutions of appropriate Boltzmann
equation are compared with those obtained using some popular approximate
methods. As an application of general results the relic density
of dark radiation in Weinberg's Higgs portal model is discussed.
|Parallel Session||Dark Matter, Astroparticle Physics|