Limits from primordial nucleosynthesis on the properties of massive neutral leptons

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Abstract
If there exist neutral leptons with masses in the range 50 eV to 5 GeV, they would have been present in thermal equilibrium in the early stages of the hot big bang. In the subsequent evolution of the universe, if their lifetime is sufficiently long their mass dominated the energy density of the universe. In this paper we consider the effect of their presence on the synthesis of elements in the early universe. Of the observed primordial abundances, we find the helium abundance to be independent of their existence, but we find the deuterium abundance to be sufficiently sensitive to allow bounds to be placed on the mass, lifetime, and decay modes of any heavy neutrinos. In particular, on the basis of present best estimates of astrophysical parameters, we reduce previous radiative lifetime bounds on the order of months to bounds on the order of hours, and expand the range of masses for which no radiatively decaying massive neutral leptons are allowed to 50 eV to 100 keV.