Radiative neutron capture on a proton at BBN energies

Abstract

The total cross section for radiative neutron capture on a proton, $np\to d\gamma$, 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 $d\gamma \to np$ cross section and the photon analyzing power for the $d\vec{\gamma }\to np$ 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 $np\to d\gamma$ cross section at some energies estimated by an R-matrix analysis. We also study the uncertainties in our estimation of the $np\to d\gamma$ cross section at relevant BBN energies and find that the estimated cross section is reliable to within $\sim$1% error.

Comments: 21 pages and 12 eps figures; 6 eps figures and 2 references added, and accepted for publication in Phys. Rev. C