Covariant spectator theory of np scattering: Isoscalar interaction currents

Using the covariant spectator theory (CST), one boson exchange models have been found that give precision fits to low-energy np scattering and the deuteron binding energy. The boson-nucleon vertices used in these models contain a momentum dependence that requires a new class of interaction currents for use with electromagnetic interactions. Current conservation requires that these new interaction currents satisfy a two-body Ward-Takahashi identity, and using principals of simplicity and picture independence, these currents can be uniquely determined. The results lead to general formulas for a two-body current that can be expressed in terms of relativistic np wave functions, , and two convenient truncated wave functions, (2) and , which contain all of the information needed for the explicit evaluation of the contributions from the interaction current. These three wave functions can be calculated from the CST bound- or scattering-state equations (and their off-shell extrapolations). A companion paper uses this formalism to evaluate the deuteron magnetic moment.