Big bang nucleosynthesis constraints on hadronically and electromagnetically decaying relic neutral particles

Big bang nucleosynthesis in the presence of decaying relic neutral particles is examined in detail. All nonthermal processes important for the determination of light-element abundance yields of H-2, H-3, He-3, He-4, Li-6, and Li-7 are coupled to the thermonuclear fusion reactions to obtain comparatively accurate results. Predicted light-element yields are compared to observationally inferred limits on primordial light-element abundances to infer constraints on the abundances and properties of relic decaying particles with decay times in the interval 0.01 sec less than or similar to tau(X)less than or similar to 10(12) sec. Decaying particles are typically constrained at early times by He-4 or H-2, at intermediate times by Li-6, and at large times by the He-3/H-2 ratio. Constraints are shown for a large number of hadronic branching ratios and decaying particle masses and may be applied to constrain the evolution of the early universe.