  During recent years, a good agreement was found between the analytical
derivation and the numerical simulation of the Landau gauge gluon and ghost
propagators. We mention the Schwinger-Dyson and Gribov-Zwanziger formalism for
the analytical work. Although the agreement between several approaches is nice,
these propagators do not correspond to the relevant physical degrees of
freedom. In the case of pure gauge theories, one should start to study the
glueball correlators. We shall try to explain why it looks like a hard
challenge to go from the unphysical to the physical propagators in the case of
the Gribov-Zwanziger theory (but similar conclusions might hold for other
approaches giving similar propagators).
