De treden van de dagen: op het nieuw millenium...

Goed, nog één keertje dan: het millennium. Vrijdag 31 december aanstaande belooft een heel bijzondere dag te worden. Een dag waarop we symbolisch een periode van 1000 jaar afsluiten. Een eigenaardig gevoel overigens voor iemand die van die duizend jaar slechts de laatste veertig heeft meegemaakt. Waarom mag ik (bij leven en welzijn uiteraard) bij de afsluitingsfeestelijkheden aanwezig zijn, terwijl veel oudgedienden die op het afgelopen millennium werkelijk een stempel hebben gedrukt niet tot de genodigden behoren

Light Fermion Mass Effects in e^+e^- -> 4 Fermions

We investigated the effect of the light fermion masses on cross sections for e^+e^- into 4 fermions in the Fermion Loop scheme defined in hep-ph/9612260, and approximations to it. The effects are found to be very small, except of course in the collinear region of single W-boson production where the electron mass acts as the cut-off.Comment: 8 pages, latex, 3 ps figs include

Stable Calculations for Unstable Particles: Restoring Gauge Invariance

We discuss theoretical and phenomenological aspects of the use of boson propagators with energy-dependent widths in predictions for high-energy scattering processes. In general, gauge invariance is violated in such calculations. We discuss several approaches to restore gauge invariance, necessary for a reliable result. The most promising method is the addition of the relevant parts of the fermionic corrections, which fulfills all Ward identities. The numerical difference between this and other approaches is studied. A number of recommendations are given for LEP2 computations.Comment: 13 pages LaTeX inc 1 figure, uses epsf, axodraw. The ps file is available as ftp://rulgm4.leidenuniv.nl/pub/preprints/stable.ps or on http://rulgm4.leidenuniv.nl/ under preprints. Added two reference

The fermion-loop scheme for finite-width effects in e^+ e^- annihilation into four fermions

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 the six-fermion (LEP2) processes e^- e^+ -> four fermions, with massless external fermions. The fermion-loop scheme consists in including all fermionic one-loop corrections in tree-level amplitudes and resumming the self-energies. We give explicit results for the unrenormalized fermionic one-loop contributions to the gauge-boson self-energies and the triple gauge-boson vertices, and perform the renormalization in a gauge-invariant way by introducing complex pole positions and running couplings. A simple effective Born prescription is presented, which allows for a relatively straightforward implementation of the fermion-loop scheme in LEP1 and LEP2 processes. We apply this prescription to typical LEP2 processes, i.e., e^- e^+ -> \mu^- \bar{\nu}_\mu u \bar{d}, e^- e^+ -> s \bar{c} u \bar{d}, and e^- e^+ -> e^- \bar{\nu}_e u \bar{d}, and give numerical comparisons with other gauge-invariance-preserving schemes in the energy range of LEP2, NLC and beyond.Comment: 49 pages, LaTeX, 2 figures, uses amssymb, axodra