Ab initio Calculation of the np → dy Radiative Capture Process

Abstract

Lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture process np → dγ, and the photo-disintegration processes γ[superscript (*)]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[subscript π] ~ 450 and 806 MeV, and are combined with pionless nuclear effective field theory to determine the amplitudes for these low-energy inelastic processes. At m[subscript π] ~ 806 MeV, using only lattice QCD inputs, a cross section σ[superscript 806 MeV] ~ 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 σ[superscript lqcd](np → dγ) = 334.9([+5.2 over -5.4]) mb is obtained at the same incident neutron speed, consistent with the experimental value of σ[superscript expt](np → dγ) = 334.2(0.5) mb.