Two-pion exchange electromagnetic current in chiral effective field theory using the method of unitary transformation

Chiral effective field theory (EFT) provides a systematic and model-independent framework to analyze low-energy hadronic processes in harmony with the spontaneously broken approximate chiral symmetry of QCD. This approach has been successfully applied to the derivation of the nuclear forces and, more recently, also hyperon-nucleon and hyperon-hyperon interactions, see Refs. [1, 2, 3] for review articles. Exchange vector and axial currents in nuclei have also been studied in the framework of chiral EFT. In their pioneering work, Park, Min and Rho applied heavy-baryon chiral perturbation theory to derive exchange axial [4] and vector [5] currents at the one-loop level for small values of the photon momentum focusing, in particular, on the axial-charge and magnetic moment operators, respectively. These calculations were carried out employing time-ordered perturbation theory to extract non-iterative contributions to the amplitude. The resulting exchange vector currents were applied within a hybrid approach to analyze magnetic moments and radiative capture cross sections of thermal neutrons on light nuclei [5, 6, 7, 8] as well as some polarization observables in radiative neutron capture on the proton [9]. For applications to various electroweak few-nucleon reactions of astrophysical interest see [10, 11, 12, 13, 14].