title: Ab initio Calculation of the np→dγ 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^{(*)}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_{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_{π}~806 MeV, using only lattice QCD inputs, a cross section σ^{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^{lqcd}(np→dγ)=334.9(+5.2-5.4) mb is obtained at the same incident neutron speed, consistent with the experimental value of σ^{expt}(np→dγ)=334.2(0.5) mb.
authors.affiliation: Department of Physics, University of Washington, Box 351560, Seattle, Washington 98195, USA.
authors.forename: Silas R
authors.initials: SR
authors.lastname: Beane
authors.affiliation: Institute for Nuclear Theory, University of Washington, Seattle, Washington 98195-1560, USA.
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Chang
authors.affiliation: Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
authors.forename: William
authors.initials: W
authors.lastname: Detmold
authors.affiliation: Department of Physics, College of William and Mary, Williamsburg, Virginia 23187-8795, USA.|Jefferson Laboratory, 12000 Jefferson Avenue, Newport News, Virginia 23606, USA.
authors.forename: Kostas
authors.initials: K
authors.lastname: Orginos
authors.affiliation: Departament d'Estructura i Constituents de la Matèria and Institut de Ciències del Cosmos, Universitat de Barcelona, Martí i Franquès 1, Barcelona, 08028, Spain.
authors.forename: Assumpta
authors.initials: A
authors.lastname: Parreño
authors.affiliation: Institute for Nuclear Theory, University of Washington, Seattle, Washington 98195-1560, USA.
authors.forename: Martin J
authors.initials: MJ
authors.lastname: Savage
authors.affiliation: Department of Physics, The City College of New York, New York, New York 10031, USA.|Graduate School and University Center, The City University of New York, New York, New York 10016, USA.|RIKEN BNL Research Center, Brookhaven National Laboratory, Upton, New York 11973, USA.
authors.forename: Brian C
authors.initials: BC
authors.lastname: Tiburzi