Local BRST invariance and the dynamical origin of the BRST condition

The BRST invariance of theories with local space-time symmetries, such as the reparametrization-invariant point-particle or d-dimensional general relativity, and of theories with local internal symmetries, like abelian and non-abelian gauge theories, can be gauged without changing the physical content of the theory, at least at the level of perturbation theory. In such a formulation the BRST condition Q=0 can be derived from the dynamics, instead of having to be imposed as an additional constraint needed to distinguish between the physical and unphysical solutions of the theory. This construction then leads to an infinite hierarchy of BRST symmetries, all with the same cohomology. (orig.)