Ronald Kleiss

Single bremsstrahlung processes in gauge theories

Abstract We find that, for QED and for SU( N ) gauge theories, single hard bremsstrahlung cross sections are remarkably simple in the ultrarelativistic limit. They can be written as a product of two factors: one factor is connected with the lowest order, elastic process, and the second one is related to the infrared factor which describes soft quantum emission. We present explicit formulae for various processes.

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.

Spinor techniques for calculating pp → W±/Z0 + jets

Abstract We present techniques which enable one to calculate quickly the amplitudes for many scattering processes in the high-energy limit. As an illustration of the method, these are applied to the diagrams for p p → V + 0, 1 or 2 jets , where V = W ± or Z 0 . The form of the results lends itself to immediate numerical evaluation.

Complete Lowest Order Calculations for Four Lepton Final States in electron-Positron Collisions

We present a Monte Carlo procedure which enables us to calculate exact cross sections for processes such as e+e− → e+e−e+e−, e+e− → e+e−μ+μ−, e+e− → μ+μ−μ+μ−, e+e− → μ+μ−τ+τ−. In particular, attention is paid to the calculation of the complete matrix element squared including all interferences and all masses and to the method by which unweighted events are generated. This scheme can also be used to calculate cross sections for e+e− → e+e− qq or e+e− → μ+μ−qq.

Radiative corrections to Bhabha scattering at high energies: (II). Hard photon corrections and Monte Carlo treatment

Abstract The cross section for single hard photon bremsstrahlung in Bhabha scattering is presented, and its main qualitative features are reviewed. Subsequently, the structure of a Monte Carlo event generator for Bhabha scattering at LEP/SLC energies, including all one-loop electroweak corrections, is discussed, in which the results on the virtual and soft photon corrections, given in the first paper of this set, are incorporated. Finally, a number of numerical results is presented.

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.

Multiple bremsstrahlung in gauge theories at high energies (II). Single bremsstrahlung

We calculate the helicity amplitudes for the QED processes e+e−→γγγ, μ+μ−γ, and e+e−γ, in the limit of vanishing fermion masses. This is done by introducing explicit polarization vectors for the radiated photons. With the same technique, we also calculate these processes taking into account Z0 exchange. Finally, we present the unpolarized cross sections in which mass terms are retained to describe the radiation of photons in directions parallel to those of the fermions.

Monte Carlo simulation of two-photon processes: II: Complete lowest order calculations for four-lepton production processes in electron-positron collisions

Program obtainable from: CPC Program Library, Queen’s University of Belfast, N. Ireland (see application form in this Method of solution issue) The 36 diagrams are divided into four groups (see ref. [11).For each group we design a separate generator in which all the Computer: Amdahl V7B or IBM 370/158 phase-space variables have been chosen such that the peaking structure is well described. The interference between the groups Programming language used: FORTRAN H-EXT * is taken care of by the assignment of a weight to the events. High speed storage required: 256 K Each event is first obtained according to one out of four approximate distributions by the corresponding generator. We No. of bits in a word: 32 account for the approximations used by imposing a weight on the events. Upon the application of a rejection algorithm we

All electroweak four-fermion processes in electron-positron collisions

Abstract This paper studies the electroweak production of all possible four-fermion states in e + e − collisions. Since the methods employed to evaluate the complete matrix elements and phase space are very general, all four-fermion final states in which the charged particles are detected can be considered. Also all kinds of experimental cuts can be imposed. With the help of the constructed event generator a large number of illustrative results is obtained, which show the relevance of backgrounds to a number of signals. For LEP 200 the W-pair signal and its background are discussed, for higher energies also Z-pair and single W and Z signals and backgrounds are presented.This paper studies the electroweak production of all possible four fermion states in e+ e- collisions. Since the methods employed to evaluate the complete matrix elements and phase space are very general, all four fermion final states in which the charged particles are detected can be considered. Also all kinds of experimental cuts can be imposed. With the help of the constructed event generator a large number of illustrative results is obtained, which show the relevance of backgrounds to a number of signals. For LEP 200 the W-pair signal and its background are discussed, for higher energies also Z-pair and single W and Z signals and backgrounds are presented.

Weight optimization in multichannel Monte Carlo

Abstract We discuss the improvement in the accuracy of a Monte Carlo integration that can be obtained by optimization of the a-priori weights of the various channels. These channels may be either the strata in a stratified-sampling approach, or the several ‘approximate’ distributions such as are used in event generators for particle phenomenology. The optimization algorithm does not require any initialization, and each Monte Carlo integration point can be used in the evaluation of the integral. We describe our experience with this method in a realistic problem, where an effective increase in program speed by almost an order of magnitude is observed.