Gauge Invariance and the Unstable Particle

It is shown how to construct exactly gauge-invariant S-matrix elements for processes involving unstable gauge particles such as the $Z^0$ boson. The results are applied to derive a physically meaningful expression for the cross-section $\sigma (e^{+}{e}^{-}\to Z^0{Z}^0)$ and thereby provide a solution to the long-standing problem of the unstable particle.