  Gauge theory correlators are potentially more singular in the infrared than
those in non-gauge theories. We determine the implications that these
singularities have on the spectrum of the theory, proving that the appearance
of generalised poles implies the existence of on-shell states with fixed mass,
but zero norm. For quantum chromodynamics these poles have direct relevance for
the confinement of coloured states. Using lattice data for the Landau gauge
gluon propagator we subsequently test for the presence of these poles,
establishing that the data is indeed consistent with such a component.
