arXiv subjects

F. A. Berends

Publications and source records attributed to F. A. Berends.

27 records · Page 2Linked to original sources

arXiv1996

Small-threshold behaviour of two-loop self-energy diagrams: two-particle thresholds

The behaviour of two-loop two-point diagrams at non-zero thresholds corresponding to two-particle cuts is analyzed. The masses involved in a cut and the external momentum are assumed to be small as compared to some of the other masses of the diagram. By employing general formulae of asymptotic expansions of Feynman diagrams in momenta and masses, we construct an algorithm to derive analytic approximations to the diagrams. In such a way, we calculate several first coefficients of the expansion. Since no conditions on relative values of the small masses and the external momentum are imposed, the threshold irregularities are described analytically. Numerical examples, using diagrams occurring in the Standard Model, illustrate the convergence of the expansion below the first large threshold.

hep-ph↗

arXiv1996

WW Cross-sections and Distributions

We present the results obtained by the "WW Cross-sections and Distributions" working group during the CERN Workshop "Physics at LEP2" (1994/1995)

hep-ph↗

arXiv1995

Anomalous four-fermion processes in electron-positron collisions

This paper studies the electroweak production of all possible four-fermion states in $e^+e^-$ collisions with non-standard triple gauge boson couplings. All $CP$ conserving couplings are considered. It is an extension of the methods and strategy, which were recently used for the Standard Model electroweak production of four-fermion final states. Since the fermions are taken to be massless the matrix elements can be evaluated efficiently, but certain phase space cuts have to be imposed to avoid singularities. Experimental cuts are of a similar nature. With the help of the constructed event generator a number of illustrative results is obtained for $W$-pair production. These show on one hand the distortions of the Standard Model angular distributions caused by either off-shell effects or initial state radiation. On the other hand, also the modifications of distributions due to anomalous couplings are presented, considering either signal diagrams or all diagrams.

hep-ph↗

arXiv1994

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

Motivated by the precision results in the electroweak theory studies of two-loopFeynman 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.

hep-ph↗

arXiv1994

Zero-threshold expansion of two-loop self-energy diagrams

An algorithm is constructed to derive a small momentum expansion for two-loop two-point diagrams in all cases where, due to the presence of physical thresholds, there are singularities at zero external momentum. The coefficients of this ``zero-threshold'' expansion are calculated analytically for arbitrary masses. Numerical examples, using diagrams occurring in the Standard Model, illustrate the convergence of the expansion below the first non-zero threshold.

hep-ph↗

arXiv1994

Calculation of two-loop self-energies in the electroweak Standard Model

Motivated by the results of the electroweak precision experiments, studies of two-loop self-energy Feynman diagrams are performed. An algebraic method for the reduction of all two-loop self-energies to a set of standard scalar integrals is presented. The gauge dependence of the self-energies is discussed and an extension of the pinch technique to the two-loop level is worked out. It is shown to yield a special case of the background-field method which provides a general framework for deriving Green functions with desirable theoretical properties. The massive scalar integrals of self-energy type are expressed in terms of generalized multivariable hypergeometric functions. The imaginary parts of these integrals yield complete elliptic integrals. Finally, one-dimensional integral representations with elementary integrands are derived which are well suited for numerical evaluation.

hep-ph↗

arXiv1994

Four fermion production in e+ e- annihilation

Four fermion processes at e+ e- colliders in a range of energy from 100 GeV to 1 TeV are discussed and examples of results obtained with an event generator presented. We also investigate the effects of the inclusion of initial state QED corrections and QCD contributions.

hep-ph↗

arXiv1994

Initial-state QED corrections to four-fermion production in e+e- collisions at LEP200 and beyond

The implementation of QED initial-state radiative corrections in the process of four-fermion production at LEP200 and higher-energy e+e- colliders is discussed. Because of the presence of charged-current processes, this is a nontrivial problem, and we compare our approach with other existing treatments. We describe the Monte Carlo algorithm used for the generation of 4-fermion events with photon bremsstrahlung. Comparison between our event generator and semi-numerical calculations are presented, as well as predictions for W and Z pair related cross sections.

hep-ph↗

arXiv1994

All electroweak four fermion processes in electron-positron collisions

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.

hep-ph↗