Covariance of Light-Front Models: Pair Current

TL;DR

This paper investigates the electromagnetic current of a spin-one two-fermion system on the light-front, highlighting the importance of pair production amplitudes and zero modes for maintaining covariance.

Contribution

It demonstrates the necessity of including pair production amplitudes and zero modes to preserve covariance in light-front calculations of composite systems.

Findings

Pair production amplitudes are essential for covariance.

Zero modes contribute significantly to the current.

Numerical examples illustrate the impact of pair terms.

Abstract

We compute the "+" component of the electromagnetic current of a composite spin-one two-fermion system for vanishing momentum transfer component $q^+=q^0+q^3$. 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.