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D. Schildknecht

Publications and source records attributed to D. Schildknecht.

At least 37 records · Page 2Linked to original sources
arXiv1999

Off-diagonal generalized vector dominance in DIS and QCD

We review the generalized vector dominance (GVD) approach to DIS at small values of the scaling variable, x. In particular, we concentrate on a recent formulation of GVD that explicitly incorporates the configuration of the gamma^* -> q qbar transition and a QCD-inspired ansatz for the (qqbar)p scattering amplitude. The destructive interference, originally introduced in off-diagonal GVD is traced back to the generic strcuture of two-gluon exchange. Asymptotically, the transverse photoabsorption cross section behaves as (ln Q^2)/Q^2, implying a logarithmic violation of scaling for F_2, while the longitudinal-to-transverse ratio decreases as 1/\ln Q^2. We also briefly comment on vector-meson production.

hep-ph↗
arXiv1999

High-Energy Cross Section for e^+e^- to W^+ W^- to 4 fermions + gamma

A recently suggested high-energy Born-form representation of the one-loop helicity amplitudes for e^+e^- to W^+W^- is supplemented by including W^+- decay and hard-photon radiation. Results for the differential and the total cross section for e^+e^- to W^+W^- to 4 fermions + gamma are given for the high-energy region of sqrt(s) > 500 GeV.

hep-ph↗
arXiv1999

Deep inelastic Scattering, QCD, and generalised vector dominance

We provide a formulation of generalised vector dominance (GVD) for low-x deep-inelastic scattering that explicitly incorporates the $γ^{\ast} \to q{\bar q}$ transition and a QCD-inspired ansatz for the $(q{\bar q})p$ forward-scattering amplitude. The destructive interference originally introduced in off-diagonal GVD is recovered in the present formulation and traced back to the generic structure of two-gluon-exchange as incorporated into the notion of colour transparency. Asymptotically, the transverse photoabsorption cross section behaves as (ln Q^2)/Q^2, implying a logarithmic violation of scaling for F_2, while the longitudinal-to-transverse ratio decreases as 1/ln Q^2.

hep-ph↗
arXiv1999

Low x_{bj} DIS, QCD and Generalized Vector Dominance

We give a brief overview on the present status of Generalized Vector Dominance as appplied to vector-meson production and the total photoabsorption cross section in the region of small x_{bj}. We comment on how GVD originates from QCD notions such as color transparency.

hep-ph↗
arXiv1998

A Simple Born-Form Approximation for e^+ e^- to W^+W^- at One Loop

A simple Born-form approximation at the one-loop level for e^+e^- to W^+ W^- at high energies is given in analytic form. The different contributions to the three invariant one-loop amplitudes, S_I^{(-)}(s,t) and S_Q^{(\pm)}(s,t), determining the Born-form helicity amplitudes are thoroughly investigated analytically and numerically. At energies above 500 GeV, the accuracy of the simple Born-form approximation for the differential production cross section is better than 1% for almost all W^+W^- production angles, independently of whether the W^+W^- polarization is summed over, or whether a longitudinal or a transverse polarization is selected for both the W^+ and the W^-.

hep-ph↗
arXiv1997

Electroweak Theory and the 1996/97 Precision Electroweak Data

We review the empirical evidence for the validity of the Standard Electroweak Theory in Nature. The experimental data are interpreted in terms of an effective Lagrangian for Z physics, allowing for potential sources of SU(2) violation and containing the predictions of the Standard Electroweak Theory as a special case. Particular emphasis is put on discriminating loop corrections due to fermion-loop vector-boson propagator corrections on the one hand, from corrections depending on the non-Abelian structure and the Higgs sector on the other hand. Results from recently obtained fits of the Higgs-boson mass are reported, yielding M_H < 430 GeV [680 GeV] at 95% C.L. based on the input of \swbar^2(\LEP+\SLD)_{`97} = 0.23152 +- 0.00023 [\swbar^2(\LEP)_{`97} = 0.23196 +- 0.00028]. The LEP2 data provide first direct experimental evidence for non-zero non-Abelian couplings among the electroweak vector bosons.

hep-ph↗
arXiv1997

On e+e- to W+W- at LEP2

We briefly discuss the first direct experimental evidence from LEP2 on non-vanishing trilinear couplings among the weak vector bosons. Subsequently we review the improved Born approximation to include radiative corrections. Provided the appropriate high-energy scale is used for the SU(2) gauge-coupling, the improved Born approximation is sufficiently accurate for all practical purposes at LEP2.

hep-ph↗
arXiv1997

Generalized Vector Dominance and low x inelastic electron-proton scattering

The HERA data on inelastic lepton proton scattering for values of the Bjorken scaling variable x < 0.05 are confronted with predictions based on Generalized Vector Dominance. Good agreement between theory and experiment is found over the full kinematic range of the squared four-momentum-transfer, Q^2, from Q^2 = 0 (photoproduction) to Q^2 < 350 GeV^2.

hep-ph↗
arXiv1997

A Simple Approximation of the One-Loop Corrected Cross Section for $e^+e^-\to W^+W^-$ at LEP 2

Using the SU(2) gauge coupling, $g_{W^\pm} (M^2_{W^\pm})$, at the high-energy scale of $M_{W^\pm}$, defined by the (theoretical value of the) leptonic W-width, rather than using the low-energy value, defined via the Fermi coupling, $G_μ$, in the Born approximation, and supplementing with Coulomb corrections and initial state radiation, errors with respect to the exact one-loop results for the differential cross section of $e^+e^-\to W^+W^-$ are below 1% at LEP 2 energies at all $W^+W^-$ production angles. A similar procedure is suggested to incorporate leading bosonic loop effects into four-fermion production in the fermion-loop scheme. The resulting accuracy below 1% is sufficient for LEP 2 experiments.

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

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.

hep-ph↗
arXiv1996

Discovering Non-Abelian Weak Couplings and an Anomalous Magnetic Dipole Moment of the $W^{\pm}$ at LEP~2

In view of the forthcoming results on $W^{\pm}$ pair production at LEP~2, we emphasize that {\em direct empirical evidence} on non-trivial properties of the weak vector bosons can be obtained with relatively limited integrated luminosity. An integrated luminosity of $10\;\mathrm{pb}^{-1}$ at $\sqrt{s}=175$ GeV will be sufficient to provide direct experimental evidence for non-vanishing self-couplings of non-Abelian type among the weak vector bosons. An integrated luminosity of $100\;\mathrm{pb}^{-1}$ at $\sqrt{s}=175$ GeV will provide direct evidence for the existence of an anomalous magnetic dipole moment of the charged vector bosons $W^{\pm}$.

hep-ph↗
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.

hep-ph↗
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.

hep-ph↗
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.

hep-ph↗