Radiative Neutron-Proton Capture in Effective Chiral Lagrangians

We calculate the cross section for the thermal $n+p\to d+\gamma$ process in chiral perturbation theory to next-to-next-to-leading order using heavy-fermion formalism. The exchange current correction is found to be $(4.5\pm 0.3)\%$ in amplitude, and the chiral perturbation at one-loop order gives the cross section ${\sigma }_{\mathrm{th}}=334\mathrm{}\pm 2$ mb, which is in agreement with the experimental value $334.2\pm 0.5\mathrm{mb}$. Together with the axial-charge transitions, this provides a strong support for the power of chiral Lagrangians for nuclear physics.