Astrophysical $S$-factors in primordial nucleosynthesis
The astrophysical $S$-factor plays an important role in extrapolating the laboratory cross section data to lower energies of relevance in nucleosynthesis calculations. Be it empirical fits or theoretical modeling of the $S$-factors, experimental information on the cross sections over a range of energies is essential to provide reliable $S$-factors and thereby the nuclear reaction rates which enter the nucleosynthesis networks. In view of the scattered and sometimes diverse data on the same reaction in literature and the databases, we present a compilation and review of available data from 318 references involving 371 experiments on the leading and sub-leading reactions in big bang nucleosynthesis (BBN). Salient features such as bumps at certain energies and a steep rise in the $S$-factor at low energies due to screening effects are found to vary in different experiments. The latter depends on the variation of targets and sometimes temperature to extract information on the same nuclear reaction. For instance, 45 cases in which the $S$-factor at low-energy could present electron screening-effect showed deviations from background values with features which are unique to each experiment, target and temperature. The extensive compilation, discussions of the relevance of each reaction in BBN and the features of the $S$-factors is followed by a presentation of empirical fits to the $S$-factors in some selected reactions of interest. This is a preprint of the following work: Carlos A. Salas and Neelima G. Kelkar, Astrophysical S-Factors in Primordial Nucleosynthesis, 2026, SpringerBriefs in Physics. It is the version of the author's manuscript prior to acceptance for publication and has not undergone editorial and/or peer review on behalf of the Publisher (where applicable). The final authenticated version is available online at: https://doi.org/10.1007/978-3-032-30466-7