This talk reports on work aimed at improving our understanding of charged states in gauge theories.Emphasis is placed on different ways of implementingthe gauge invariance of physical states. QED perturbative calculations are used to stress that gauge invariance is not enough: only a subset of such states have a direct physical significance. In QCD a non-perturbative obstruction means that quarks and gluons cannot form physical states and as such these particles are not truely observables. This sets a fundamental limit on the constituent quark model.

Publication Date


Publication Title

Published in the proceedings of the workshop on Quantum Chromodynamics: Collisions, Confinement and Chaos, Editors H.M. Fried and B. Mueller, World Scientific 1997

Organisational Unit

School of Engineering, Computing and Mathematics