Covariance of light-front models: pair current Original Research Article

We compute the + component, i.e., j+=j0+j3, of the electromagnetic current of a composite spin-one two-fermion system for vanishing momentum transfer component q+=q0+q3. In particular, we extract the nonvanishing pair production amplitude on the light-front. It is a consequence of the longitudinal zero momentum mode, contributing to the light-front current in the Breit-frame. The covariance of the current is violated, if such pair terms are not included in its matrix elements. We illustrate our discussion with some numerical examples.