How do constituent quarks arise in QCD? Perturbation theory and the infra-red

We motivate the use of dressed charges by arguing that such objects are needed to describe, e.g., constituent quarks and, in general, physical charged states in gauge theories. We give a short introduction to dressings in both QED and QCD. We put special emphasis on the infra-red properties of a moving dressed charge. To be more precise, we demonstrate that the one loop propagator of a relativistic dressed charge can be renormalized in the mass shell scheme with no infra-red divergence showing up.