arXiv subjects

S. Dittmaier

Publications and source records attributed to S. Dittmaier.

124 records · Page 7Linked to original sources

arXiv1996

Implications of Electroweak Precision Data on Bounds on the Higgs-Boson Mass

Carefully analyzing the dependence of the bounds on the Higgs-boson mass $\MH$ derived from the observables $\swbar^2(\LEP)$, $\swbar^2(\SLD)$, $\Gl$, $\MW$, $\GT$, $\Gh$, as well as $\Rb$ and $\Rc$, and considering the uncertainties in $α(\MZ^2)$ and $α_s(\MZ^2)$, we find that a stronger bound than $40\GeV\lsim\MH\lsim 1\TeV$ at the $1σ$ level can hardly be deduced at present, even if the experimental information on the top-quark mass $\Mt$ from direct production is taken into account.

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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)

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arXiv1996

On the Observation of Bosonic Loop Corrections in Electroweak Precision Experiments

We investigate the structure of the experimentally significant electroweak bosonic loop corrections to the leptonic weak mixing angle, the leptonic Z-boson decay width, and the W-boson mass. It is shown that the bosonic corrections that have a sizable effect at the present level of experimental accuracy are directly related to the use of the Fermi constant G_F as input parameter for analyzing the LEP observables. Indeed, if the (theoretical value of the) leptonic width of the W boson is used as input parameter instead of the low-energy parameter G_F determined from muon decay, fermion-loop corrections are sufficient for compatibility between theory and experiment.

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arXiv1996

Electroweak Radiative Corrections to AA -> W+W-

We discuss the complete virtual and soft-photonic O(alpha) corrections to AA -> W+W- within the electroweak Standard Model for arbitrary polarized photons. In the on-shell renormalization scheme for fixed M_W no leading corrections associated with the running of alpha or heavy top-quark and Higgs-boson masses occur. The corrections turn out to be of the order of 10%, but can become much larger where the lowest-order cross-sections are suppressed.

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arXiv1995

Which Bosonic Loop Corrections are Tested in Electroweak Precision Measurements?

The nature of the electroweak bosonic loop corrections to which current precision experiments are sensitive is explored. The set of effective parameters $\De x$, $\De y$, and $\eps$, which quantify SU(2) violation in an effective Lagrangian, is shown to be particularly useful for this purpose. The standard bosonic corrections are sizable only in the parameter $\De y$, while $\De x$ and $\eps$ are sufficiently well approximated by the pure fermion-loop prediction. By analyzing the contributions to $\De y$ it is shown that the bosonic loop corrections resolved by the present precision data are induced by the change in energy scale between the low-energy process muon decay and the energy scale of the LEP1 observables. If the (theoretical value of the) leptonic width of the W boson is used as input parameter instead of the Fermi constant $\GF$, no further bosonic loop corrections are necessary for compatibility between theory and experiment.

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arXiv1995

Radiative Corrections to $γγ\to t \bar t$ in the Electroweak Standard Model

The cross-section for $γγ\to t \bar t$ with arbitrary polarized photons is calculated within the electroweak Standard Model including the complete virtual and soft-photonic $O(α)$ corrections. We present a detailed numerical discussion of the radiative corrections with particular emphasis on the purely weak corrections. These are usually of the order of 1--10\% for energies up to 1 TeV. For unpolarized or equally polarized photons they reach almost 10\% close to threshold. The large corrections cannot be traced back to a universal origin like the running of $α$ or the $ρ$-parameter. Apart from the energy region around the Higgs resonance $(γγ\to H^*\to t\bar t)$ the weak corrections are widely independent of the Higgs-boson mass.

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arXiv1995

Radiative Corrections to $γγ\to W^+ W^-$ in the Electroweak Standard Model

The cross-section for $γγ\to W^+ W^-$ with arbitrary polarized photons and W bosons is calculated within the electroweak Standard Model including the complete virtual and soft-photonic ${\cal O}(α)$ corrections. We present a detailed numerical discussion of the complete radiative corrections and an analytical investigation of the leading corrections. It turns out that in the on-shell renormalization scheme for fixed $M_W$ no leading corrections associated with the running of $α$ or heavy top-quark and Higgs-boson masses occur. The corrections are typically of the order of 10\%. They reach, however, larger values where the lowest-order cross-sections are suppressed.

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arXiv1995

Refined Analysis of the Electroweak Precision Data

We refine our recent analysis of the electroweak precision data at the \PZO\ pole by including the hadronic decay modes of the \PZO. Within the framework of an effective Lagrangian we parametrize $SU(2)$ violation by the additional process-specific parameters $\De y_ν$, $\De\yh$, and $\De\yb$ (for the $\PZOν\barν$, $\PZO\Pq\bar\Pq$, and $\PZO\Pb\bar\Pb$ vertices) together with the previously introduced parameters $\De x$, $\De y$, and $\eps$. We find that a six-parameter analysis of the experimental data is indeed feasible, and it is carried out in addition to a four-parameter fit for $\De x$, $\De y$, $\eps$, and $\De\yb$ only. We reemphasize that the experimental data have become sensitive to the (combined) magnitude of the vertex corrections at the $\PWp\Pl\barν$ ($\PWmν\bar\Pl$) and $\PZO\Pl\bar\Pl$ vertices, $\De y$, which is insensitive to the notion of the Higgs mechanism but dependent on the non-Abelian, trilinear vector-boson coupling. Full explicit analytical results for the standard one-loop predictions for the above-mentioned parameters are given, and the leading two-loop top-quark effects are included. The analytic formluae for the analysis of the experimental data in terms of the parameters $\De x$, $\De y$ etc.\ are presented in order to encourage experimentalists to persue such an analysis by themselves with future data.

hep-ph↗

arXiv1994

Probing the Higgs mechanism via $γγ\to W^+W^-$

We investigate the sensitivity of the reaction $γγ\to W^+W^-$ to the Higgs sector based on the complete one-loop corrections in the minimal Standard Model and the gauged non-linear $σ$-model. While this sensitivity is very strong for the suppressed cross-section of equally polarized photons and longitudinal W bosons, it is only marginal for the dominant mode of transverse polarizations. The corrections within the $σ$-model turn out to be UV-finite in accordance with the absence of $\log\MH$ terms in the Standard Model with a heavy Higgs boson.

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arXiv1994

Application of the Background-Field Method to the electroweak Standard Model

Application of the background-field method yields a gauge-invariant effective action for the electroweak Standard Model, from which simple QED-like Ward identities are derived. As a consequence of these Ward identities, the background-field Green functions are shown to possess very desirable theoretical properties. The renormalization of the Standard Model in the background-field formalism is studied. A consistent on-shell renormalization procedure retaining the full gauge symmetry is presented. The structure of the counterterms is shown to greatly simplify compared to the conventional formalism. A complete list of Feynman rules for the Standard Model in the background-field method is given for arbitrary values of a quantum gauge parameter including all counterterms necessary for one-loop calculations.

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arXiv1994

Green functions from a gauge-invariant effective action for the electroweak Standard Model

Application of the background-field method to the electroweak Standard-Model yields a gauge-invariant effective action giving rise to simple Ward identities. We find that in the background-field `t Hooft-Feynman gauge the resulting vertex functions are exactly those that are obtained using the pinch technique. Thus, the background-field method provides a general framework that generalizes the pinch technique directly and uniquely to arbitrary Green functions and arbitrary orders of perturbation theory. Moreover, the desirable properties of the pinch-technique vertex functions hold for arbitrary gauge parameters and have a simple explanation within the background- field method.

hep-ph↗

arXiv1994

On the Role of the Higgs Mechanism in Present Electroweak Precision Tests

Based on the observables $\MW$, $Γ_l$, $\bar\sw^2(\MZ^2)$, we evaluate the parameters $Δx, Δy$ and $\varepsilon$ at one-loop level within an electroweak massive vector-boson theory, which does not employ the Higgs mechanism. The theoretical results are consistent with the experimental ones on $Δx$, $Δy$, $\varepsilon$. The theoretical prediction for $Δy$ coincides with the standard-model one (apart from numerically irrelevant terms which vanish for $\MH\to\infty$). Non-renormalizability only affects $Δx$ and $\varepsilon$, which differ from the standard-model results by the replacement $\log\MH\to\logΛ$ for a heavy Higgs mass, $\MH$ (where $Λ$ denotes an effective UV cut-off).

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arXiv1994

Gauge Invariance of Green Functions: Background-Field Method versus Pinch Technique

Application of the background-field method to QCD and the electroweak Standard Model yields gauge-invariant effective actions giving rise to simple Ward identities. Within this method, we calculate the quantities that have been treated in the literature using the pinch technique. Putting the quantum gauge parameter equal to one, we recover the pinch-technique results as a special case of the background-field method. The one-particle-irreducible Green functions of the background-field method fulfil for arbitrary gauge parameters the desirable theoretical properties that have been noticed within the pinch technique. Therefore the background-field formalism provides a general framework for the direct calculation of well-behaved Green functions. Within this formalism, the pinch technique appears as one of arbitrarily many equivalent possibilities.

hep-ph↗

arXiv1994

On the Significance of the Electroweak Precision Data

We elaborate on a recently suggested effective Lagrangian for charged-current and neutral-current electroweak interactions which in comparison with the standard electroweak theory contains three free parameters $Δx, Δy$, $\varepsilon$ which quantify different sources for violations of $SU(2)$ symmetry. Within the standard $SU(2)_I \times U(1)_Y$ electroweak theory, we present both exact and very much refined approximate analytical one-loop expressions for these parameters in terms of the canonical input, $G_μ, \MZ$, $\alpz$, the top-quark mass, $\Mt$, and the Higgs-boson mass, $\MH$. We reemphase the importance of discriminating between the {\it empirically well-known purely fermionic} (vacuum polarization) contributions to $Δx, Δy$, $\varepsilon$ and the {\it empirically unknown bosonic} ones with respect to present and future electroweak precision tests. The parameters $Δx$ and $\varepsilon$ are hardly affected by standard bosonic corrections, while the full one-loop results for $Δy$ differ appreciably from the ones obtained by taking into account fermion loops only. A detailed comparison with the experimental data on $\MWpm/\MZ$, $\bar\sw^2$, $Γ_l$ shows that these data start to become accurate enough to be sensitive to standard (bosonic) contributions to $Δy$ beyond fermion loops.

hep-ph↗

arXiv1993

Full O(alpha) Radiative Corrections to High-Energy Compton Scattering

Using computer-algebraic methods we derive compact analytical expressions for the virtual electroweak radiative corrections to polarized Compton scattering. Moreover the helicity amplitudes for double Compton scattering, which prove to be extremely simple in terms of Weyl-van der Waerden spinor products, are presented for massless electrons. The inclusion of a finite electron mass is described, too. Finally numerical results both for the purely photonic and the full O(alpha) electroweak corrections, which turn out to be of the order of 5-10%, are discussed for energies ranging from 10GeV to 2TeV.

hep-ph↗

arXiv1993

Gauge-Boson Production in Electron-Photon Collisions

We summarize results for the processes $e^-γ\to νW^-, e^-Z, e^-γ$ within the electroweak Standard Model. We discuss the essential features of the corresponding lowest-order cross-sections and present numerical results for the unpolarized cross-sections including the complete $O(α)$ virtual, soft-photonic, and hard-photonic corrections. While at low energies the weak corrections are dominated by the leading universal corrections, at high energies we find large, non-universal corrections, which arise from vertex and box diagrams involving non-Abelian gauge couplings.

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