An effective Lagrangian approach for unstable particles

We propose a novel procedure For handling processes that involve unstable intermediate particles. By using gauge-invariant effective Lagrangians it is possible to perform a gauge-in variant resummation of (arbitrary) self-energy effects. For instance, gauge-invariant tree-level amplitudes can be constructed with the decay widths of the unstable particles properly included in the propagators. In these tree-level amplitudes modified vertices are used, which contain extra gauge-restoring terms prescribed by the effective Lagrangians. We discuss the treatment of the phenomenologically important unstable particles, like the top quark, the W- and Z-bosons, and the Higgs boson, and derive the relevant modified Feynman rules explicitly. (C) 2000 Elsevier Science B.V. All rights reserved.