Electromagnetic structure of A=2 and 3 nuclei and the nuclear current operator

Different models for conserved two- and three-body electromagnetic currents are constructed from two- and three-nucleon interactions, using either meson-exchange mechanisms or minimal substitution in the momentum dependence of these interactions. The connection between these two different schemes is elucidated. A number of low-energy electronuclear observables, including (i) $np$ radiative capture at thermal neutron energies and deuteron photodisintegration at low energies, (ii) $nd$ and $pd$ radiative capture reactions, and (iii) isoscalar and isovector magnetic form factors of $^3$H and $^3$He, are calculated in order to make a comparative study of these models for the current operator. The realistic Argonne $v_{18}$ two-nucleon and Urbana IX or Tucson-Melbourne three-nucleon interactions are taken as a case study. For $A$=3 processes, the bound and continuum wave functions, both below and above deuteron breakup threshold, are obtained with the correlated hyperspherical-harmonics method. Three-body currents give small but significant contributions to some of the polarization observables in the $^2$H($p,\gamma$)$^3$He process and the $^2$H($n,\gamma$)$^3$H cross section at thermal neutron energies. It is shown that the use of a current which did not exactly satisfy current conservation with the two- and three-nucleon interactions in the Hamiltonian was responsible for some of the discrepancies reported in previous studies between the experimental and theoretical polarization observables in $pd$ radiative capture.