Radiative neutron capture on a proton at big-bang nucleosynthesis energies

The total cross section for radiative neutron capture on a proton, np -> d gamma, is evaluated at big-bang nucleosynthesis (BBN) energies. The electromagnetic transition amplitudes are calculated up to next-to-leading-order within the framework of pionless effective field theory with dibaryon fields. We also calculate the d gamma -> np cross section and the photon analyzing power for the d gamma -> np process from the amplitudes. The values of low-energy constants that appear in the amplitudes are estimated by a Markov Chain Monte Carlo analysis using the relevant low-energy experimental data. Our result agrees well with those of other theoretical calculations except for the np -> d gamma cross section at some energies estimated by an R-matrix analysis. We also study the uncertainties in our estimation of the np -> d gamma cross section at relevant BBN energies and find that the estimated cross section is reliable to within similar to 1% error.