  We point out that the relic densities of singlet (sterile) neutrinos of
interest in viable warm and cold dark matter scenarios, depend on the
characteristics of the QCD transition in the early universe. In the most
promising of these dark matter scenarios the production of the singlets occurs
at or near the QCD transition. Since production of the singlets, their
dilution, and the disappearance of weak scatterers occur simultaneously, we
calculate these processes contemporaneously to obtain accurate predictions of
relic sterile neutrino mass density. Therefore, a determination of the mass and
superweak mixing of the singlet neutrino through, for example, its radiative
decay, along with a determination of its contribution to the critical density,
can provide insight into the finite-temperature QCD transition.
