THE FERMION-LOOP SCHEME FOR FINITE-WIDTH EFFECTS IN E(-) ANNIHILATIONINTO 4 FERMIONS()E()

We describe the gauge-invariant treatment of the finite-width effects of W and Z bosons in the fermion-loop scheme and its application to th e six-fermion (LEP2) processes e(-)e(+) --> four fermions, with massle ss external fermions. The fermion-loop scheme consists in including al l fermionic one-loop corrections in tree-level amplitudes and resummin g the self-energies. We give explicit results for the unrenormalized f ermionic one-loop contributions to the gauge-boson self-energies and t he triple gauge-boson vertices, and perform the renormalization in a g auge-invariant way by introducing complex pole positions and running c ouplings. A simple effective Born prescription is presented, which all ows for a relatively straightforward implementation of the fermion-loo p scheme in LEP1 and LEP2 processes. We apply this prescription to typ ical LEP2 processes, i.e. e(-)e(+) --> mu(-)<(nu)over bar(mu)>mu (d) o ver bar, e(-)e(+) --> <s(c)over bar><u(d)over bar>, and e(-)e(+) --> e (-)<(nu)over bar><u(d)over bar>, and give numerical comparisons with o ther gauge-invariance-preserving schemes in the energy range of LEP2, NLC and beyond. (C) 1997 Published by Elsevier Science B.V.