Radiativeα-capture cross sections from realistic nucleon-nucleon interactions and variational Monte Carlo wave functions

We report the first calculations of cross sections for the radiative capture reactions ${}^3\mathrm{H}(\alpha ,\gamma {)}^7\mathrm{Li}$ and ${}^3\mathrm{He}(\alpha ,\gamma {)}^7\mathrm{Be}$ below 2 MeV that use wave functions derived from realistic nucleon-nucleon interactions by the variational Monte Carlo technique. After examining several small corrections to the dominant $E1\mathrm{}$ operator, we find energy dependences for the low-energy S factors that agree reasonably with experimental measurements. There is no contradiction with the previous theoretical understanding of these processes, but the zero-energy derivative of the ${}^3\mathrm{H}(\alpha ,\gamma {)}^7\mathrm{Li}$ S factor is smaller than that in most models. While this method can, in principle, predict cross section normalizations, the normalizations of our results are mostly too low.