BIG-BANG NUCLEOSYNTHESIS CONSTRAINTS ON LIGHT SCALARS AND THE 17 KEV NEUTRINO HYPOTHESIS

In this paper we present a detailed discussion of the constraints that the primordial abundances of light elements impose on the interactions of light scalars which are relativistic at the time of nucleosynthesis. We discuss the implications of our results for those ''17 keV neutrino'' models which invoke the presence of Goldstone bosons in order to provide a fast decay or annihilation mechanism for the heavy neutrinos. Although our results can be applied to a variety of cases, we use for our discussion the ''invisible-majoron'' scenario. Unless an unnatural tuning of the parameters is considered, we find that the scale of spontaneous breaking of lepton number is bounded to be above 19 GeV. While the possibility of a fast decay of the heavy neutrinos into majorons is allowed, we can confidently exclude the annihilation into majorons as a means of depleting the cosmological abundance of stable 17 keV neutrinos.