F.A. Berends

EXCALIBUR - a Monte Carlo program to evaluate all four-fermion processes at LEP 200 and beyond*

A Monte Carlo program is presented that computes all four-fermion processes in e+e - annihilation. QED initial state corrections and QCD contributions are included. Fermions are taken to be massless, allowing a very fast evaluation of the matrix element. A systematic, modular and self-optimizing strategy has been adopted for the Monte Carlo integration, which serves also as an example for further event generators in high energy particle physics.

Radiative corrections to muon pair and quark pair production in electron-positron collisions in the Z0 region

Abstract A calculation of first-order radiative corrections to the process e + e − → μ + μ − is presented, which is in particular applicable to the Z 0 region. The emphasis is on a detailed treatment of hard photon effects, which affect the size of the corrections in the Z 0 region considerably. The technique used is that of a Monte Carlo simulation of μ + μ − and μ + μ − γ events. In an appendix the generalization to quark pair production is presented.

Electroweak radiative corrections to e+e− → W+W−

The cross section for e+e− → W+W− with arbitrary polarizations of the leptons and bosons is calculated in the standard electroweak model including the complete one-loop virtual and soft-photon bremsstrahlung corrections. The relevant analytic results are listed and the influence of the radiative corrections on the unpolarized cross section is discussed. We find moderate corrections at LEP II energies. The corrections increase continuously with energy up to − 50% of the Born cross section at 1 TeV.

On relations between multi-gluon and multi-graviton scattering

Abstract From a relation between tree amplitudes of closed and open strings it becomes possible to express a mulit-graviton amplitude into a quadratic combination of multi-gluon sub-amplitudes. Since up to six external particles the graviton and gluon amplitudes are known, we can explicitly demonstrate the correctness of the relations in these cases. For a special helicity configuration the n-graviton amplitude is given on the basis of the known n-gluon amplitudes. The soft graviton behaviour is discussed.

On the production of a W and jets at hadron colliders

Abstract In this paper the evaluation of matrix elements for a vector boson decaying into n partons ( n ⩽ 6) is presented. For this purpose recursive techniques and Weyl-van der Waerden spinor calculus are used. By appropriately crossing partons the amplitudes can be used to describe the production of a W and jets. The four-jet case is of particular interest as background to interesting physics signals. Numerical results are given for present and future accelerator energies. Also the signal versus background question for top quark search is briefly discussed.

Analytical and numerical methods for massive two-loop self-energy diagrams

Abstract Motivated by the precision results in the electroweak theory studies of two-loop Feynman diagrams are performed. Specifically this paper gives a contribution to the knowledge of massive two-loop self-energy diagrams in arbitrary and especially four dimensions. This is done in three respects: firstly results in terms of generalized, multivariable hypergeometric functions are presented giving explicit series for small and large momenta. Secondly the imaginary parts of these integrals are expressed as complete elliptic integrals. Finally one-dimensional integral representations with elementary functions are derived. They are very well suited for the numerical evaluations.

Large-angle Bhabha scattering

Abstract The paper presents a calculation of the total cross section for large-angle Bhabha scattering. Numerical results for LEP/SLC energies are presented. We impose cuts on both the angles and the energies of the outgoing electron and positron and on their acollinearity. The first-order QED corrections are taken into account completely, the higher-order QED corrections are incorporated in the leading log approach. Pure weak corrections are also included. The description of the Z-peak in Bhabha scattering is now brought to a level of precision comparable to the pure s-channel processes.

Exact expressions for processes involving a vector boson and up to five partons

Exact expressions are derived for the matrix elements of parton process relevant for jet production in e+e−, γγ, e−P and PP collisions. For the latter the formulae describe the production of a vector boson in conjunction with up to three jets. The possibility to evaluate cross sections in an arbitrary dimension and for massive quarks is kept open for the most relevant processes. Explicit results for helicity amplitudes of massless partons are given for all processes. The general structure of the process V→qq + n gluons emerges from the explicit calculations up to n = 3. A systematic use is made of recursive techniques, the abelian part of gluon field strengths and spinor calculus.

Radiative corrections to pair production of unstable particles: Results for E+E− → 4 fermions

Abstract Radiative corrections to processes that involve the production and subsequent decay of unstable particles are complex due to various theoretical and practical problems. The so-called double-pole approximation offers a way out of these problems. This method is applied to the reaction e+e− → W+W− → 4 fermions, which allows us to address all the key issues of dealing with unstable particles, like gauge invariance, interactions between different stages of the reaction, and overlapping resonances. Within the double-pole approximation the complete O (α) electroweak corrections are evaluated for this off-shell W-pair production process. Examples of the effect of these corrections on a number of distributions are presented. These comprise mass and angular distributions as well as the photon-energy spectrum.

Radiative corrections to Bhabha scattering and mupair production from the hadronic vacuum polarization

Abstract The known first order radiative corrections to Bhabha scattering and mupair production are extended to include the hadronic vacuum polarization. For the evaluation, the measured total cross-section for e + e − → hadrons together with some assumptions on its high-energy behaviour is used. The results show that the inclusion of the hadronic vacuum polarization in the radiative corrections is required for precision tests of quantumelectrodynamics at high energies.