Constituent Quarks from QCD: Perturbation Theory and the Infra-red
  Systematic approaches to building up gauge invariant descriptions of charged fields, such as electrons or quarks, are described. Physically relevant descriptions must then be singled out from a multiplicity of possibilities and to this end we give a physical interpretation of one description. Perturbative calculations which back up this interpretation are outlined. A non-perturbative obstruction to observing an isolated quark is reported. This sets the limits of the constituent quark model. 
E. Bagan, M. Lavelle, D. McMullan, B. Fiol, N. Roy