Effect of interacting particles on primordial nucleosynthesis

We modify the standard model for big-bang nucleosynthesis to allow for the presence of a generic particle species, i.e., one which maintains good thermal contact with either the photons or the light-neutrino species throughout the epoch of primordial nucleosynthesis. The production of D, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ is calculated as a function of the mass, degrees of freedom, and spin statistics of the generic particle. We show that in general, the effect of an additional generic species cannot simply be parametrized as the equivalent number of additional light-neutrino species. The presence of generic particles also affects the predicted value for the neutrino-to-photon temperature ratio.