Constraints on active-sterile neutrino mixing from primordial abundances

Abstract

Starting from the calculation of the neutrino distribution function, as a function of the active-sterile neutrino mixing parameters, we calculate primordial abundances of light nuclei (D, ${}^{4}$He, and ${}^{7}$Li) using a modified version of Kawano's code. The active-sterile neutrino mixing parameters and the baryon-to-photon ratio $({\ensuremath{\eta}}_{B})$ enter the calculation as free parameters. By using the observational data of D, ${}^{4}$He, and ${}^{7}$Li, we set constraints on ${\ensuremath{\eta}}_{B}$ and on the active-sterile neutrino mixing parameters. It is found that the value for the baryon-to-photon ratio depends strongly on the inclusion of ${}^{7}$Li data in the sample, while the value for the mixing angles remain small.