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 qq̄ combination together with the outgoing electron and positron. The dependence on the photon virtualities Q12 and Q22 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.

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 (q) over bar lowlying S-1(0), (3) P-j and (1) D-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, chi(c2), chi(c1) --> gammaJ/psi the details are given. Several numerical results are presented as well. (C) 2002 Elsevier Science B.V. All rights reserved.

A fully massive Monte Carlo program to compute all four-fermion processes in electron positron collisions, including Higgs boson production, is presented. Leading higher order effects are discussed and included.

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.

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 100MeV.

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.

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 chi(cl) eta(c) (2S), and eta(b) (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. (C) 1998 Elsevier Science B.V.

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.

A study is made of the feasibility of maximum likelihood fits to determine MW 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 MW determination for semileptonic final states in W-pair production show the feasibility of the method. For TGCs one result is presented as an illustration.

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.

Analytic results for the threshold and pseudothreshold values of the sunset diagram with arbitrary masses are obtained in terms of dilogarithms of ratios of the masses.

In this paper the general form of scattering amplitudes for massless particles with equal spins s (ss --> ss) or unequal spins (s(a)s(b) --> 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. (C) 1997 Elsevier Science B.V.

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. Numerical results are presented, in particular the effects on the W line-shape. These effects are of the order of a per cent. The validity of the presented calculations starts a few widths above the W-pair threshold. 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.

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.

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

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.

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

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.

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.