Ab initio Calculation of the np → dγ Radiative Capture Process

Lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture process np -> d gamma, and the photo-disintegration processes gamma(*)d -> np. In nuclear potential models, such contributions are described by phenomenological meson-exchange currents, while in the present work, they are determined directly from the quark and gluon interactions of QCD. Calculations of neutron-proton energy levels in multiple background magnetic fields are performed at two values of the quark masses, corresponding to pion masses of m(pi) similar to 450 and 806MeV, and are combined with pionless nuclear effective field theory to determine the amplitudes for these low-energy inelastic processes. At m(pi) similar to 806 MeV, using only lattice QCD inputs, a cross section sigma(806 MeV) similar to 17 mb is found at an incident neutron speed of v = 2,200 m/s. Extrapolating the short-distance contribution to the physical pion mass and combining the result with phenomenological scattering information and one-body couplings, a cross section of sigma(lqcd)(np -> d gamma) = 334.9((+5.2)(-5.4)) mb is obtained at the same incident neutron speed, consistent with the experimental value of sigma(expt)(np -> d gamma) = 334.2(0.5) mb.