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F. A. Berends

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

At least 19 recordsLinked to original sources
arXiv2001

Two-photon mediated resonance production in e+e- collisions: cross sections and density matrices

Earlier described model amplitudes are used in this paper to evaluate both cross sections and density matrices for two-photon mediated resonance production in e^+e^- collisions. All 25 q\bar{q} low-lying ^1S_0, ^3P_J and ^1D_2 resonances can thus be treated. Two independent methods are described to obtain the resonance production density matrices and cross sections. These density matrices combined with a resonance decay density matrix give the detailed angular distributions of the resonance decay products. For two particular decays, χ_{c2},χ_{c1}\toγJ/ψthe details are given. Several numerical results are presented as well.

hep-ph↗
arXiv2001

GaGaRes: A Monte Carlo generator for resonance production in two-photon physics

In this paper a Monte Carlo generator, GaGaRes, is presented which can be used to describe two-photon resonance production in e^+e^- collisions. The program can generate the five lowest-lying C=+1 meson states of any q\bar{q} combination together with the outgoing electron and positron. The dependence on the photon virtualities Q_1^2 and Q_2^2 is fully taken into account. The program also generates the density matrices of the resonance, which form an essential tool in the description of the decay of the resonances. Furthermore, the program is applicable for all tagging conditions.

hep-ph↗
arXiv2000

Four-fermion production in electron-positron collisions with NEXTCALIBUR

We introduce a new, fully massive, Monte Carlo program to compute all four-fermion processes in electron-positron collisions, including Higgs production. We outline our strategy for the matrix element evaluation, the phase space generation and the implementation of the leading higher order effects, and show, where available, comparisons with existing results.

hep-ph↗
arXiv2000

An effective Lagrangian approach for unstable particles

We propose a novel procedure for handling processes that involve unstable intermediate particles. By using gauge-invariant effective Lagrangians it is possible to perform a gauge-invariant resummation of (arbitrary) self-energy effects. For instance, gauge-invariant tree-level amplitudes can be constructed with the decay widths of the unstable particles properly included in the propagators. In these tree-level amplitudes modified vertices are used, which contain extra gauge-restoring terms prescribed by the effective Lagrangians. We discuss the treatment of the phenomenologically important unstable particles, like the top-quark, the $W$- and $Z$-bosons, and the Higgs-boson, and derive the relevant modified Feynman rules explicitly.

hep-ph↗
arXiv1999

One-loop QCD interconnection effects in pair production of top quarks

We calculate the one-loop non-factorizable QCD corrections to the production and decay of pairs of top quarks at various collider experiments. These non-factorizable corrections interconnect the different production and decay stages of the off-shell top-pair production processes. This in particular affects the invariant-mass distributions of the off-shell top quarks, resulting in a shift of the maximum of the distorted Breit-Wigner distributions. Although the non-factorizable corrections can be large, the actual shift in the mass as determined from the peak position of the corrected Breit-Wigner line-shape is below $100\MeV$.

hep-ph↗
arXiv1998

Radiative corrections to pair production of unstable particles: results for e^+e^- --> 4 fermions

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^{-} \to W^{+}W^{-} \to 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 $\OO(α)$ 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.

hep-ph↗
arXiv1998

WEXTER and ERAFITTER: two programs to fit M_W at LEP2 using the best measurable kinematical variables

In this paper, we present two programs to fit M_W at LEP2 using the best measurable kinematical variables. The theoretical probabilities of observing the final-state kinematical configurations are computed by integrating over the quantities that are not well measured. Therefore, an event-by-event kinematical reconstruction is avoided. M_W is then determined through a maximum likelihood fit.

hep-ph↗
arXiv1998

Final-state radiation and line-shape distortion in resonance pair production

In this letter it is shown how final-state QED corrections to the production of a pair of resonances can distort the line shape of such a resonance in a sizeable way. This effect depends on the definition of the line shape and can reach up to 30%, depending on the final state. The mechanism is first displayed for a particular case of ZZ production, for which an exact and approximate treatment can be given. The approximate method is then applied to W-pair production. In addition some simple rules of thumb are given for accurately estimating the characteristic distortion effects, like the mass shift and peak reduction.

hep-ph↗
arXiv1997

On the determination of M_W and TGCs in W-pair production using the best measured kinematical variables

A study is made of the feasibility of maximum likelihood fits to determine M_W and triple gauge boson couplings using only those experimental kinematical variables that are well measured. A computational tool to calculate theoretical probabilities for those kinematical variables is discussed and then applied to samples of unweighted events produced by an event generator. Detailed results on the M_W determination for semileptonic final states in W-pair production show the feasibility of the method. For TGCs one result is presented as an illustration.

hep-ph↗
arXiv1997

Meson--photon transition form factors and resonance cross-sections in e^+ e^- collisions

Meson--photon--photon transition form factors for S-, P-, and D-wave states are calculated, the meson being treated as a non-relativistic heavy-quark--antiquark pair. The full dependence on both photon virtualities is included. Cross-section formulas for charge-conjugation even mesons with J^P = 0^-, 0^+, 1^+, 2^+, and 2^- in electron--positron collisions are presented and numerical results for LEP energies are given. In particular, we find two-photon event rates for χ_{\rc 1}, η_{\rc}(2S), and η_{\rb}(1S) within reach of LEP. With minor modifications to incorporate SU(3)-flavour breaking we estimate rates for 18 light mesons as well, based on the observation that their two-photon decay widths agree remarkably well with measured data. Finally we point out that e^+ e^- cross sections for 1^+ states do not vanish at low Q^2, the Landau--Yang suppression factors of the two-photon cross sections being compensated by the photon propagators.

hep-ph↗
arXiv1997

Event Generators for WW Physics

The report summarizes the results of the activities of the Working Group on Event Generators for WW Physics at CERN during 1995.

hep-ph↗
arXiv1997

Non-factorizable corrections to W-pair production: methods and analytic results

In this paper we present two methods to evaluate non-factorizable corrections to pair-production of unstable particles. The methods are illustrated in detail for W-pair-mediated four-fermion production. The results are valid a few widths above threshold, but not at threshold. One method uses the decomposition of $n$-point scalar functions for virtual and real photons, and can therefore be generalized to more complicated final states than four fermions. The other technique is an elaboration on a method known from the literature and serves as a useful check. Applications to other processes than W-pair production are briefly mentioned.

hep-ph↗
arXiv1997

Non-factorizable corrections to W-pair production

In this paper we study the non-factorizable QED corrections to W-pair-mediated (charged-current) four-fermion production in electron-positron collisions. A brief account of the obtained analytical results is given. They turn out to be different from the ones published in the literature. For the first time numerical results are presented, in particular the effects on the W line-shape. These effects are of the order of a per cent. Applying the same methods to ZZ- or ZH-mediated four-fermion production, the non-factorizable $O(α)$ corrections to the Z or H line-shape vanish.

hep-ph↗
arXiv1997

On the Construction of Scattering Amplitudes for Spinning Massless Particles

In this paper the general form of scattering amplitudes for massless particles with equal spins s ($s s \to s s$) or unequal spins ($s_a s_b \to s_a s_b$) are derived. The imposed conditions are that the amplitudes should have the lowest possible dimension, have propagators of dimension $m^{-2}$, and obey gauge invariance. It is shown that the number of momenta required for amplitudes involving particles with s > 2 is higher than the number implied by 3-vertices for higher spin particles derived in the literature. Therefore, the dimension of the coupling constants following from the latter 3-vertices has a smaller power of an inverse mass than our results imply. Consequently, the 3-vertices in the literature cannot be the first interaction terms of a gauge-invariant theory. When no spins s > 2 are present in the process the known QCD, QED or (super) gravity amplitudes are obtained from the above general amplitudes.

hep-th↗