Electromagnetic structure of A=2 and 3 nuclei in chiral effective field theory

Background: The A=2 and 3 form factors are among the observables of choice for testing models of nuclear interactions and associated electromagnetic charge and current operators. Here we investigate the validity of the chiral-effective-field-theory (χEFT) approach to describe the strong-interaction dynamics in these few-nucleon systems and their response to electromagnetic probes. Purpose: The objectives of the present work are twofold. The first is to address and resolve some of the differences present in independent, χEFT derivations up to one loop, recently appearing in the literature, of the nuclear charge and current operators. The second objective is to provide a complete set of χEFT and hybrid predictions for the structure functions and tensor polarization of the deuteron, for the charge and magnetic form factors of 3He and 3H, and for the charge and magnetic radii of these few-nucleon systems. Methods: The calculations use wave functions derived from either chiral or conventional two- and three-nucleon potentials and Monte Carlo methods to evaluate the relevant matrix elements. Results: In reference to the two objectives mentioned earlier, we find that (i) there are no differences between the χEFT magnetic dipole operator at one loop derived in our formalism and that obtained by Kölling with the unitary transformation method and (ii) there is excellent agreement between theory and experiment for the static properties and elastic form factors of these A=2 and 3 nuclei up to momentum transfers q≲2.0-2.5 fm^-1. A complete analysis of the results is provided. Conclusions: Nuclear χEFT provides a very satisfactory description of the isoscalar and isovector charge and magnetic structure of the A=2 and 3 nuclei at low momentum transfers q23m_π. In particular, contributions from two-body charge and current operators are crucial for bringing theory into close agreement with experiment. At higher q values the present χEFT predictions are similar to those obtained in the hybrid approach, as well as in older studies based on the conventional meson-exchange picture, and fail to reproduce the measured A=2 and 3 form factors in the diffraction region.