2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/47042It is shown how to construct exactly gauge-invariant S-matrix elements for processes involving unstable gauge particles such as the $W$ and $Z^0$ bosons. The results are applied to derive a physically meaningful expression for the cross-section $σ(e^+e^-\to Z^0Z^0)$ and thereby provide a solution to the long-standing problem of the unstable particle. The problem of maintaining QED gauge-invariance in the process $e^+e^-\to\barν_e e^-W^+\to\barν_e e^-u\bar d$ is examined.8 pages LaTeX. Uses axodraw.sty and procsla.sty. Talk presented at The Higgs Puzzle --- What can we learn from LEP II, LHC, NLC and FNC, Ringberg Castle, Tegernsee, Germany, 9--13 December, 1996High Energy Physics - Phenomenologytext