Gauge Invariance and the Unstable Particle

Abstract

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.

Comments: 8 pages LaTeX. Uses aipproc.sty and epsfig.sty. Talk presented at 1st Latin American Symposium on High-energy Physics, SILAFAE-I, Merida, November 1--5, 1996