The cross section for $n\overrightarrow{p}d\gamma$ is calculated at energies relevant to big-bang nucleosynthesis using the recently developed effective field theory that describes the two-nucleon sector. The $E1\mathrm{}$ amplitude is computed up to ${\mathrm{N}}^3\mathrm{LO}$ and depends only upon nucleon-nucleon phase shift data. In contrast, the $M1\mathrm{}$ contribution is computed up to next-to-leading order, and the four-nucleon-one-magnetic-photon counterterm that enters is determined by the cross section for cold neutron capture. The uncertainty in the calculation for nucleon energies up to $E\sim 1\mathrm{MeV}$ is estimated to be $\lesssim 4\%.$

• Received 27 July 1999

DOI:https://doi.org/10.1103/PhysRevC.60.065205