arXiv subjects

S. Dittmaier

Publications and source records attributed to S. Dittmaier.

At least 109 records · Page 6Linked to original sources

arXiv1999

W-pair production at future e+e- colliders: precise predictions from RACOONWW

We present numerical results for total cross sections and various distributions for e+e- --> WW --> 4f(+gamma) at a future 500GeV linear collider, obtained from the Monte Carlo generator RACOONWW. This generator is the first one that includes O(alpha) electroweak radiative corrections in the double-pole approximation completely. Owing to their large size the corrections are of great phenomenological importance.

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arXiv1999

Four-fermion production with RACOONWW

RACOONWW is an event generator for e+e- --> WW --> 4fermions(+gamma) that includes full tree-level predictions for e+e- --> 4f and e+e- --> 4f+gamma as well as O(alpha) corrections to e+e- --> 4f in the so-called double-pole approximation. We briefly sketch the concept of the calculation on which this generator is based and present some numerical results.

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arXiv1999

O(alpha) corrections to e+e- --> WW --> 4fermions(+gamma): first numerical results from RACOONWW

First numerical results of the Monte Carlo generator RACOONWW for e+e- --> WW --> 4fermions(+gamma) in the electroweak Standard Model are presented. This event generator is the first one that includes O(alpha) electroweak radiative corrections in the double-pole approximation completely. We briefly describe the strategy of the calculation and give numerical results for total cross sections, including CC03, and various distributions.

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arXiv1999

Radiative corrections to e^+e^- -> WW -> 4 fermions

The structure of the double-pole approximation for the O(alpha) corrections to e^+e^- -> WW -> 4 fermions is described, and some results are presented. Moreover, results on full tree-level predictions for $e^+e^- -> WW -> 4 fermions + gamma are given.

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arXiv1999

Predictions for all processes e^+e^- -> 4 fermions + gamma

The complete matrix elements for e^+ e^- -> 4f and e^+ e^- -> 4f + gamma are calculated in the Electroweak Standard Model for polarized massless fermions. The matrix elements for all final states are reduced to a few compact generic functions. Monte Carlo generators for e^+ e^- -> 4f and e^+ e^- -> 4f + gamma are constructed. We compare different treatments of the finite widths of the electroweak gauge bosons; in particular, we include a scheme with a complex gauge-boson mass that obeys all Ward identities. The detailed discussion of numerical results comprises integrated cross sections as well as photon-energy distributions for all different final states.

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arXiv1998

Production of Light Fermion-Antifermion Pairs in gamma gamma Collisions

The $O(α)$ corrections to $γγ--> f \bar f$ in the Standard Model are calculated for arbitrary, light fermions $f$. The relevant analytical results are listed in a form that is appropriate for practical applications, and numerical results for integrated cross sections are discussed. The corresponding QED corrections are generally of the order of some per mille for arbitrary energies. The weak corrections to $γγ--> e^-e^+$ are negligible below the electroweak scale, reach the per-cent level at a few hundred GeV and grow to about -10% at 2TeV. The weak corrections to $u \bar u$ and $d \bar d$ production have a shape similar to the one for $e^-e^+$, but they are larger by factors $\sim 1.4$ and $\sim 3$, respectively.

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arXiv1998

Higgs Radiation off Top Quarks in e^+e^- Collisions

The strength of the Yukawa coupling of the leptons and quarks with the Higgs boson is uniquely determined by the Higgs mechanism for generating masses in the Standard Model. The top-Higgs coupling can be measured directly in the process e^+e^- --> t tbar H. This process therefore provides a fundamental test of the Higgs mechanism. Extending earlier analyses carried out in the Born approximation, we have determined the cross section for this process including QCD corrections, which turn out to be important.

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arXiv1998

Two-loop QCD corrections to Higgs-pair production at the LHC

The calculation of the next-to-leading order QCD corrections to the production of a neutral Higgs-boson pair is briefly summarized. The dominant production mechanism is gluon fusion via top-quark loops, so that the QCD corrections can be considered, to good approximation, in the limit of a heavy top-quark mass.

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arXiv1998

Complete O(alpha) QED corrections to polarized Compton scattering

The complete QED corrections of O(alpha) to polarized Compton scattering are calculated for finite electron mass and including the real corrections induced by the processes e^- gamma -> e^- gamma gamma and e^- gamma -> e^- e^- e^+. All relevant formulas are listed in a form that is well suited for a direct implementation in computer codes. We present a detailed numerical discussion of the O(alpha)-corrected cross section and the left-right asymmetry in the energy range of present and future Compton polarimeters, which are used to determine the beam polarization of high-energetic e^+- beams. For photons with energies of a few eV and electrons with SLC energies or smaller, the corrections are of the order of a few per mille. In the energy range of future e^+e^- colliders, however, they reach 1-2% and cannot be neglected in a precision polarization measurement.

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arXiv1998

Neutral Higgs-Boson Pair Production at Hadron Colliders: QCD Corrections

Neutral Higgs-boson pair production provides the possibility of studying the trilinear Higgs couplings at future high-energy colliders. We present the QCD corrections to the gluon-initiated processes in the limit of a heavy top quark in the loops and the Drell-Yan-like pair production of scalar and pseudoscalar Higgs particles. The pp cross sections are discussed for LHC energies within the Standard Model and its minimal supersymmetric extension. The QCD corrections are large, enhancing the total cross sections significantly.

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arXiv1998

Further numerical results on non-factorizable corrections to e+e- --> 4fermions

Numerical results on non-factorizable corrections to e+e- --> WW --> 4fermions with semi-leptonic and hadronic final states, as well as to e+e- --> ZZ --> 4fermions, are presented. The corrections turn out to be small in comparison to the experimental uncertainty of LEP2, but they might compete with the expected accuracy at future e+e--colliders.

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arXiv1998

Non-factorizable photonic corrections to ee->WW->4fermions

We study the non-factorizable corrections to W-pair-mediated four-fermion production in ee annihilation in double-pole approximation. We show how these corrections can be combined with the known corrections to the production and the decay of on-shell W-boson pairs, and how the full off-shell Coulomb singularity is included. Moreover, we find that the actual form of the real non-factorizable corrections depends on the parametrization of phase space, more precisely, on the definition of the invariant masses of the resonant W bosons. For the usual parametrization the full analytical results for the non-factorizable corrections are presented. Our analytical and numerical results for the non-factorizable corrections agree with a recent calculation, which was found to differ from a previous one. The detailed numerical discussion covers the invariant-mass distribution, various angular distributions, and the lepton-energy distribution for leptonic final states.

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arXiv1997

Approximations for W-Pair Production at Linear-Collider Energies

We determine the accuracy of various approximations to the O(alpha) corrections for on-shell W-pair production. While an approximation based on the universal corrections arising from initial-state radiation, from the running of alpha, and from corrections proportional to m_t^2 fails in the Linear-Collider energy range, a high-energy approximation improved by the exact universal corrections is sufficiently good above about 500GeV. These results indicate that in Monte Carlo event generators for off-shell W-pair production the incorporation of the universal corrections is not sufficient and more corrections should be included.

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arXiv1996

Radiative Corrections to ZZ->ZZ in the Electroweak Standard Model

The cross-section for ZZ->ZZ with arbitrarily polarized Z bosons is calculated within the electroweak Standard Model including the complete O(alpha) corrections. We show the numerical importance of the radiative corrections and elaborate its characteristic features. The treatment of the Higgs-boson resonance is discussed in different schemes including the S-matrix-motivated pole scheme and the background-field method. The numerical accuracy of the equivalence theorem is investigated by comparing the cross-sections for purely longitudinal Z bosons obtained from the equivalence theorem and from the complete calculation. In this context the full O(alpha) corrections are also confronted with the enhanced corrections of O(alpha M_H^2/s_w^2 M_W^2), which were frequently used in the literature.

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arXiv1996

The fermion-loop scheme for finite-width effects in e^+ e^- annihilation into four fermions

We describe the gauge-invariant treatment of the finite-width effects of W and Z bosons in the fermion-loop scheme and its application to the six-fermion (LEP2) processes e^- e^+ -> four fermions, with massless external fermions. The fermion-loop scheme consists in including all fermionic one-loop corrections in tree-level amplitudes and resumming the self-energies. We give explicit results for the unrenormalized fermionic one-loop contributions to the gauge-boson self-energies and the triple gauge-boson vertices, and perform the renormalization in a gauge-invariant way by introducing complex pole positions and running couplings. A simple effective Born prescription is presented, which allows for a relatively straightforward implementation of the fermion-loop scheme in LEP1 and LEP2 processes. We apply this prescription to typical LEP2 processes, i.e., e^- e^+ -> μ^- \barν_μu \bar{d}, e^- e^+ -> s \bar{c} u \bar{d}, and e^- e^+ -> e^- \barν_e u \bar{d}, and give numerical comparisons with other gauge-invariance-preserving schemes in the energy range of LEP2, NLC and beyond.

hep-ph↗

arXiv1996

Implications of the Electroweak Precision Data: a 1996 Update

The most reliable prediction for the Higgs-boson mass, M_H, is obtained in a fit to the leptonic Z-resonance observables Gamma_l and swbar^2 combined with the W-boson mass, M_W, and top-quark mass, m_t, measurements. The corresponding bounds on M_H are independent of potential uncertainties related to R_b, R_c, and alpha_s(M_Z^2), and they are not significantly further improved by including also the experimental information on the inclusive (hadronic and total) Z-boson decays. At the 1sigma level, we obtain M_H<360GeV using swbar^2(LEP+SLD) and M_H<540GeV using swbar^2(LEP). Our analysis in terms of effective parameters confirms previous conclusions with increased accuracy. In the mass parameter, Delta x, and the mixing parameter, epsilon, pure fermion loops are sufficient, while for the coupling parameter, Delta y, (M_H-insensitive) bosonic contributions are essential for consistency with experiment, thus providing indirect empirical evidence for the non-Abelian structure of the theory.

hep-ph↗

arXiv1996

The background-field formulation of the electroweak Standard Model

The application of the background-field method to the electroweak Standard Model and its virtues are reviewed. Special emphasis is directed to the Ward identities that follow from the gauge invariance of the background-field effective action. They are compatible with on-shell renormalization and imply a decent behavior of the background-field vertex functions. Via the usual construction of connected Green functions they transfer to Ward identities for connected Green functions which, in distinction to the conventional formalism, remain exactly valid in finite orders of perturbation theory even if a Dyson summation of self-energies (within a systematic use of one-particle irreducible building blocks) is performed. Finally, we comment on the interplay between gauge invariance and gauge-parameter (in-)dependence of vertex and Green functions and the uniqueness of resummation procedures.

hep-ph↗

arXiv1996

How reliably can the Higgs-Boson Mass be predicted from Electroweak Precision Data?

From the LEP precision data and the measurement of the W-boson mass, upon excluding the observables Rb, Rc in a combined fit of the top-quark mass, Mt, and the Higgs-boson mass, MH, within the Standard Model, we find the weak 1sigma bound of MH<900GeV. Stronger upper bounds on MH, sometimes presented in the literature, rely heavily on the inclusion of Rb in the data sample. Upon including Rb, the quality of the fit drastically decreases, and by carefully analyzing the dependence of the fit results on the set of experimental input data we conclude that these stronger bounds are not reliable. Moreover, the stronger bounds on MH are lost if the deviation between theory and experiment in Rb is ascribed to contributions of new physics. Replacing swbar^2(LEP) by the combined value swbar^2(LEP+SLD) in the data sample leads to a bound of MH<430GeV at the 1sigma level. The value of swbar^2(SLD) taken alone, however, gives rise to fit results for MH which are in conflict with MH<65.2GeV from direct searches.

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