arXiv subjects

A. Denner

Publications and source records attributed to A. Denner.

At least 55 records · Page 3Linked to original sources

arXiv2006

Two-loop electroweak next-to-leading logarithmic corrections to massless fermionic processes

We consider two-loop leading and next-to-leading logarithmic virtual corrections to arbitrary processes with external massless fermions in the electroweak Standard Model at energies well above the electroweak scale. Using the sector-decomposition method and alternatively the strategy of regions we calculate the mass singularities that arise as logarithms of Q^2/MW^2, where Q is the energy scale of the considered process, and 1/εpoles in D=4-2εdimensions, to one- and two-loop next-to-leading logarithmic accuracy. The derivations are performed within the complete electroweak theory with spontaneous symmetry breaking. Our results indicate a close analogy between the form of two-loop electroweak logarithmic corrections and the singular structure of scattering amplitudes in massless QCD. We find agreement with the resummation prescriptions that have been proposed in the literature based on a symmetric SU(2) \times U(1) theory matched with QED at the electroweak scale and provide new next-to-leading contributions proportional to ln(MZ^2/MW^2).

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arXiv2006

Precision calculations for the Higgs decays H-->ZZ/WW-->4leptons

Extending earlier work, we provide predictions obtained with the Monte Carlo generator PROPHECY4f for the decays H-->ZZ/WW-->4l including the complete electroweak O(alpha) corrections and some higher-order improvements. The gauge-boson resonances are described in the complex-mass scheme. Here, particular attention is paid to a comparison of different final states with identical charges, such as e+e-mu+mu- and mu+mu-mu+mu-.

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arXiv2006

Precise predictions for the Higgs-boson decay H -> W W/Z Z -> 4 leptons

The decay of the Standard Model Higgs boson into four leptons via a virtual W-boson or Z-boson pair is one of the most important decay modes in the Higgs-boson search at the LHC. We present the complete electroweak radiative corrections of O(α) to these processes, including improvements beyond O(α) originating from heavy-Higgs effects and final-state radiation. The intermediate W- and Z-boson resonances are described (without any expansion or on-shell approximation) by consistently employing complex mass parameters for the gauge bosons (complex-mass scheme). The corrections to partial decay widths typically amount to some per cent and increase with growing Higgs mass M_H, reaching about 8% at M_H \sim 500 GeV. For not too large Higgs masses (M_H <\sim 400 GeV) the corrections to the partial decay widths can be reproduced within <\sim 2% by simple approximations. For angular distributions the corrections are somewhat larger and distort the shapes. For invariant-mass distributions of fermion pairs they can reach several tens of per cent depending on the treatment of photon radiation. The discussed corrections have been implemented in a Monte Carlo event generator called PROPHECY4F.

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arXiv2006

The complex-mass scheme for perturbative calculations with unstable particles

Perturbative calculations with unstable particles require the inclusion of their finite decay widths. A convenient, universal scheme for this purpose is the complex-mass scheme. It fully respects gauge-invariance, is straight-forward to apply, and has been successfully used for the calculation of various tree-level processes and of the electroweak radiative corrections to e+ e- -> 4f and H -> 4f.

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arXiv2006

Les Houches Physics at TeV Colliders 2005, Standard Model and Higgs working group: Summary report

This Report summarises the activities of the "SM and Higgs" working group for the Workshop "Physics at TeV Colliders", Les Houches, France, 2-20 May, 2005. On the one hand, we performed a variety of experimental and theoretical studies on standard candles (such as W, Z, and ttbar production), treating them either as proper signals of known physics, or as backgrounds to unknown physics; we also addressed issues relevant to those non-perturbative or semi-perturbative ingredients, such as Parton Density Functions and Underlying Events, whose understanding will be crucial for a proper simulation of the actual events taking place in the detectors. On the other hand, several channels for the production of the Higgs, or involving the Higgs, have been considered in some detail. The report is structured into four main parts. The first one deals with Standard Model physics, except the Higgs. A variety of arguments are treated here, from full simulation of processes constituting a background to Higgs production, to studies of uncertainties due to PDFs and to extrapolations of models for underlying events, from small-$x$ issues to electroweak corrections which may play a role in vector boson physics. The second part of the report treats Higgs physics from the point of view of the signal. In the third part, reviews are presented on the current status of multi-leg, next-to-leading order and of next-to-next-to-leading order QCD computations. Finally, the fourth part deals with the use of Monte Carlos for simulation of LHC physics.

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arXiv2006

Electroweak Corrections to e+e- --> 4 fermions

The calculation of the full electroweak O(alpha) corrections to the charged-current four-fermion production processes e+e- --> nu_tau tau+ mu- anti-nu_mu, u anti-d mu- anti-nu_mu, and u anti-d s anti-c is briefly reviewed. The calculation is performed using the complex-mass scheme for the gauge-boson resonances. The evaluation of the occurring one-loop tensor integrals, which include 5- and 6-point functions, requires new techniques. The effects of the complete O(alpha) corrections to the total cross section and to the production-angle distribution are discussed and compared to predictions based on the double-pole approximation, revealing that the latter approximation is not sufficient to fully exploit the potential of a future linear collider in an analysis of W-boson pairs at high energies.

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arXiv2006

Techniques for one-loop tensor integrals in many-particle processes

We briefly sketch the methods for a numerically stable evaluation of tensor one-loop integrals that have been used in the calculation of the complete electroweak one-loop corrections to $\Pep\Pem\to4 $fermions. In particular, the improvement of the new methods over the conventional Passarino--Veltman reduction is illustrated for some 4-point integrals in the delicate limits of small Gram (and other kinematical) determinants.

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arXiv2005

Reduction schemes for one-loop tensor integrals

We present new methods for the evaluation of one-loop tensor integrals which have been used in the calculation of the complete electroweak one-loop corrections to e+ e- -> 4 fermions. The described methods for 3-point and 4-point integrals are, in particular, applicable in the case where the conventional Passarino-Veltman reduction breaks down owing to the appearance of Gram determinants in the denominator. One method consists of different variants for expanding tensor coefficients about limits of vanishing Gram determinants or other kinematical determinants, thereby reducing all tensor coefficients to the usual scalar integrals. In a second method a specific tensor coefficient with a logarithmic integrand is evaluated numerically, and the remaining coefficients as well as the standard scalar integral are algebraically derived from this coefficient. For 5-point tensor integrals, we give explicit formulas that reduce the corresponding tensor coefficients to coefficients of 4-point integrals with tensor rank reduced by one. Similar formulas are provided for 6-point functions, and the generalization to functions with more internal propagators is straightforward. All the presented methods are also applicable if infrared (soft or collinear) divergences are treated in dimensional regularization or if mass parameters (for unstable particles) become complex.

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arXiv2005

Electroweak corrections to Wγand Zγproduction at the LHC

We have calculated the electroweak O(α) corrections to the processes p p -> Z γ+ X -> ν_l \barν_l γ+ X, p p -> Z γ+ X -> l \bar{l} γ+ X, and p p -> W γ+ X -> ν_l \bar{l} γ+ X at the LHC, with l=e,μand ν_l=ν_e,ν_μ,ν_τ. The virtual corrections are evaluated in leading-pole approximation, whereas the real corrections are taken into account exactly. These corrections are implemented into a Monte Carlo generator which includes both phase-space slicing and subtraction to deal with soft and collinear singularities. We present numerical results for total cross sections as well as for experimentally interesting distributions. Applying typical LHC cuts, the electroweak corrections are of the order of -5% for the total cross sections and exceed -20% for observables dominated by high centre-of-mass energies of the partonic processes.

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arXiv2005

Electroweak Corrections to Four-Fermion Production in e+e- Annihilation

The recently completed calculation of the full electroweak O(alpha) corrections to the charged-current four-fermion production processes e+e- --> nu_tau tau+ mu- anti-nu_mu, u anti-d mu- anti-nu_mu, and u anti-d s anti-c is briefly reviewed. The calculation is performed using complex gauge-boson masses, supplemented by complex couplings to restore gauge invariance. The evaluation of the occurring one-loop tensor integrals, which include 5- and 6-point functions, requires new techniques. The effects of the complete O(alpha) corrections to the total cross section and to some differential cross sections of physical interest are discussed and compared to predictions based on the double-pole approximation, revealing that the latter approximation is not sufficient to fully exploit the potential of a future linear collider in an analysis of W-boson pairs at high energies.

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arXiv2004

Logarithmic electroweak corrections to gauge-boson pair production at the LHC

We have studied the effects of the complete logarithmic electroweak O(alpha) corrections on the production of vector-boson pairs WZ, ZZ, and WW at the LHC. These corrections are implemented into a Monte Carlo program for pp -> 4f (+γ) with final states involving four or two leptons using the double-pole approximation. We numerically investigate purely leptonic final states and find that electroweak corrections lower the predictions by 5-30% in the physically interesting region of large di-boson invariant mass and large angle of the produced vector bosons.

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arXiv2004

An algorithm for the high-energy expansion of multi-loop diagrams to next-to-leading logarithmic accuracy

We present an algorithm to compute arbitrary multi-loop massive Feynman diagrams in the region where the typical energy scale \sqrt{s} is much larger than the typical mass scale M, i.e. s>>M^2, while various different energy and mass parameters may be present. In this region we perform an asymptotic expansion and, using sector decomposition, we extract the leading contributions resulting from ultraviolet and mass singularities, which consist of large logarithms log(s/M^2) and 1/εpoles in D=4-2εdimensions. To next-to-leading accuracy, at L loops all terms of the form α^L ε^{-k} log^j(s/M^2) with j+k=2L and j+k=2L-1 are taken into account. This algorithm permits, in particular, to compute higher-order next-to-leading logarithmic electroweak corrections for processes involving various kinematical invariants of the order of hundreds of GeV and masses M_W \sim M_Z \sim M_H \sim M_t of the order of the electroweak scale, in the approximation where the masses of the light fermions are neglected.

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arXiv2004

Physical renormalization condition for the quark-mixing matrix

We investigate the renormalization of the quark-mixing matrix in the Electroweak Standard Model. We show that the corresponding counterterms must be gauge independent as a consequence of extended BRS invariance. Using rigid SU(2)_L symmetry, we proof that the ultraviolet-divergent parts of the invariant counterterms are related to the field renormalization constants of the quark fields. We point out that for a general class of renormalization schemes rigid SU(2)_L symmetry cannot be preserved in its classical form, but is renormalized by finite counterterms. Finally, we discuss a genuine physical renormalization condition for the quark-mixing matrix that is gauge independent and does not destroy the symmetry between quark generations.

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arXiv2004

Electroweak corrections to e+ e- -> f anti-f H

Some of the most interesting Higgs-production processes at future e+ e- colliders are of the type e+ e- -> f anti-f H. We present a calculation of the complete O(alpha) corrections to these processes in the Standard Model for final-state neutrinos and top quarks. Initial-state radiation beyond O(alpha) at the leading-logarithmic level as well as QCD corrections are also included. The electroweak corrections turn out to be sizable and reach the order of +/-10% and will thus be an important part of precise theoretical predictions for future e+ e- colliders.

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arXiv2003

Electroweak corrections to e+ e- -> nu anti-nu H and e+ e- -> t anti-t H

The most interesting Higgs-production processes at future e+ e- colliders belong to the process class e+ e- -> f anti-f H. We study the full O(alpha) corrections to this reaction in the Standard Model for neutrinos and top quarks in the final state. Leading higher-order corrections from initial-state radiation and QCD corrections are also taken into account. Although cancellations between the different kinds of corrections occur, the full corrections are of the order of +/-10% and thus important ingredients in the theoretical predictions for future e+ e- colliders.

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arXiv2003

Radiative corrections to Higgs-boson production in association with top-quark pairs at e+ e- colliders

We have calculated the complete O(alpha) and O(alpha_s) radiative corrections to the Higgs-production process e+ e- -> t anti-t H in the Standard Model. This process is particularly interesting for the measurement of the top-quark Yukawa coupling at a future e+ e- collider. The calculation of the O(alpha) corrections is described in some detail including, in particular, the treatment of the soft and collinear singularities. The discussion of numerical results focuses on the total cross section as well as on angular and energy distributions of the outgoing particles. The electroweak corrections turn out to be sizable and can reach the order of +/-10%. They result from cancellations between electromagnetic, fermionic, and weak bosonic corrections, each of which are of the order of +/-10%.

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arXiv2003

Electroweak radiative corrections to e+ e- -> t anti-t H

We have calculated the complete electroweak O(alpha) radiative corrections to the Higgs-boson production process e+ e- -> t anti-t H in the electroweak Standard Model. Initial-state radiation beyond O(alpha) is included in the structure-function approach. The calculation of the corrections is briefly described, and numerical results are presented for the total cross section. Both the photonic and the genuine weak corrections reach the order of about 10% or even more and show a non-trivial dependence on the Higgs-boson mass and on the scattering energy. We compare our results with two previous calculations that obtained differing results at high energies.

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arXiv2003

Electroweak radiative corrections to single Higgs-boson production in e+e- annihilation

We have calculated the complete electroweak O(alpha) radiative corrections to the single Higgs-boson production processes e+ e- --> nu_l anti-nu_l H (l=e,mu,tau) in the electroweak Standard Model. Initial-state radiation beyond O(alpha) is included in the structure-function approach. The calculation of the corrections is briefly described, and numerical results are presented for the total cross section. In the G_mu scheme, the bulk of the corrections is due to initial-state radiation, which affects the cross section at the level of -7% at high energies and even more in the ZH threshold region. The remaining bosonic and fermionic corrections are at the level of a few per cent. The confusing situation in the literature regarding differing results for the fermionic corrections to this process is clarified.

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