Mass and width of unstable gauge bosons

It has been proposed that the mass of the Z0 boson should be defined as the real part, and the width of the Z0 as the imaginary part of the complex pole Mc, for the reason that Mc is gauge invariant. This definition is only useful if we can formulate both, the S-matrix in the non-perturbative energy region around the Z0 resonance and Mc, in a gauge invariant way up to some order in perturbation theory. By considering an infinitesimal change of the gauge breaking term, we show this indeed to be the case, and we demonstrate that the gauge invariance of Mc is a consequence of a Ward identity. (orig.)