Abstract
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 $^{2}\mathrm{H}$, $^{3}\mathrm{H}$, $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, $^{6}\mathrm{Li}$, and $^{7}\mathrm{Li}$ 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 $0.01\text{ }\text{ }\mathrm{sec}\ensuremath{\lesssim}{\ensuremath{\tau}}_{X}\ensuremath{\lesssim}{10}^{12}\text{ }\text{ }\mathrm{sec}$. Decaying particles are typically constrained at early times by $^{4}\mathrm{He}$ or $^{2}\mathrm{H}$, at intermediate times by $^{6}\mathrm{Li}$, and at large times by the $^{3}\mathrm{He}/^{2}\mathrm{H}$ 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.
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