Primordial Nucleosynthesis with Decaying Particles. II. Inert Decays

We investigate the effect on primordial nucleosynthesis of a nonrelativistic particle X, which decays out of equilibrium into relativistic decay products which do not interact with the thermal radiation background ("inert" decays). We parameterize our results in terms of the X particle lifetime τ and the density parameter rmx, where rmx is the X particle mass and r is the ratio of X number density to photon number density prior to nucleosynthesis, The results of primordial nucleosynthesis are examined for the parameter range τ= 10⁻² - 10⁴, rmx = 10⁻²MeV. We determine the values of τ and rmx which can be excluded on the basis of primordial nucleosynthesis. This excluded region is well approximated by (rmx/MeV)(τ/s)1/2 < 2.7 for τ < 1 s, and rmx < 0.4 MeV for τ > 10³, with a smooth transition between these two regimes. We discuss the spectrum of the decay products.