np → dγ for Big - Bang Nucleosynthesis

The cross section for np → dγ is calculated at energies relevant to bigbang nucleosynthesis using the recently developed effective field theory that describes the two-nucleon sector. The E1 amplitude is computed up to N3LO and depends only upon nucleon-nucleon phase shift data. In contrast, the M1 contribution is computed up to NLO, and the four-nucleon-one-magneticphoton counterterm that enters is determined by the cross section for cold neutron capture. The uncertainty in the calculation for nucleon energies up to E ∼ 1 MeV is estimated to be ∼ 4%.