arXiv subjects
Publications and source records attributed to Dieter Schildknecht.
The transverse size of q q-bar fluctuations of the longitudinal photon is reduced relative to the transverse size of q q-bar fluctuations of the transverse photon. This implies R(W2, Q2) = 0.375 or, equivalently, FL/F2 = 0.27 at x<<0.1 and Q2 sufficiently large, while R(W2, Q2) = 0.5, if this effect is not taken into account. Forthcoming experimental data from HERA will allow to test this prediction.
The transverse size of q q-bar fluctuations of the longitudinal photon is reduced relative to the transverse size of q q-bar fluctuations of the transverse photon. This implies R(W^2, Q^2) = 0.375 or, equivalently, F_L(W^2,Q^2)/F_2(W^2,Q^2) = 0.27 for x = Q^2/W^2 << 1 and Q^2 sufficiently large, while R(W^2, Q^2) = 0.5, if this effect is not taken into account. Forthcoming experimental data from HERA will allow to test this prediction.
Assuming helicity independence for q-qbar scattering in the color-dipole picture, or, equivalently proportionality of sea quark and gluon distributions, we find R(W^2,Q^2) = 0.5 (approximately) at large Q^2, where R(W^2,Q^2) denotes the ratio of the longitudinal and transverse photoabsorption cross sections. The forthcoming direct measurements of R(W^2,Q^2) allow one to test the underlying hypotheses.
Our approach to low-x deep inelastic scattering based on the ell-perp factorization of perturbative QCD (the color-dipole picture) yields parameter-free absolute predictions for J/Psi production. The connection of J/Psi production to the low-x saturation scale and to the gluon structure function is clarified.
At low $x \cong Q^2/W^2 << 1$, in deep inelastic scattering, the photon fluctuates into a $q \bar q$ vector state that interacts via two gluons with the proton. The energy dependence is determined by the saturation scale, in our approach given by $Λ^2_{sat} (W^2) \sim (W^2)^{C_2}$. Imposing DGLAP evolution, we find $C_2^{theory} = 0.27$ in agreement with the model-independent analysis of the HERA data. Different values of the exponent $C_2$ are correlated with different ratios of the longitudinal to transverse structure function. This stresses the need for experiments to separate longitudinal and transverse contributions in deep inelastic scattering.
Historically vector-meson physics arose along two different paths to be reviewed in Sections 1 and 2. In Section 3, the phenomenological consequences will be discussed with an emphasis on those aspects of the subject matter relevant in present-day discussions on deep inelastic scattering in the diffraction region of low values of the Bjorken variable.
Consistency of the previously suggested color-dipole representation of deep-inelastic scattering (DIS_) and vector-meson production at low x with DGLAP evolution allows one to predict the exponent of the W^2 dependence of the saturation scale Lambda_sat^2(W^2) ~ (W^2)^C-2. One finds C_2^theory = 0.27 in agreement with the model-independent analysis of the experimental data from HERA on deep-inelastic electron scattering.
Based on the two-gluon-exchange dynamical mechanism for deeply inelastic scattering at low x ~= Q^2/W^2 <<1, we stress the intimate connection between the total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson electroproduction. A simple expression for the cross section for deeply virtual Compton scattering is derived. Parameter-free predictions are obtained for deeply-virtual Compton forward scattering and vector-meson forward production, once the parameters in the total virtual photoabsorption cross section are determined in a fit to the experimental data on deeply inelastic scattering. Our predictions are compared with the experimental data from HERA.
The QCD-based generalized vector dominance-color-dipole picture (GVD-CDP) provides a coherent picture of low-x DIS, deeply virtual Compton scattering and light as wel as heavy vector-meson production.
The results on e+e- --> W+W- --> 4f(+gamma) obtained by different groups are compared with each other. Differences in the results for the total cross section of up to about 0.6% are traced back to different ways of implementing the double-pole approximation.
We show that the experimental data for the total virtual-photon proton cross section, σ_{γ^*p} (W^2, Q^2), for x_{bj} \lsim 0.1 lie on a universal curve, when plotted against η= (Q^2 + m^2_0)/ Λ^2(W^2), where Λ^2 (W^2) = C_1 (W^2+W^2_0)^{C_2} is determined by the parameters m^2_0, C_1, C_2 and W^2_0. The observed scaling law follows from the generalized-vector-dominance/color-dipole picture of low-x deep inelastic scattering.
We review the results on representing the differential cross section for W-pair produciton, including W decay and hard-photon bremsstrahlung, in terms of a Born-form approximation of fairly simple analytic form. The result of the Born-form approximation are compared with full-one-loop results. The emphasis is on the energy range of future $e^+e^-$ (or $μ^+μ^-$) colliders.
Including destructively interfering off-diagonal transitions of diffraction-dissociation type, we arrive at a formulation of GVD for exclusive vector-meson production in terms of a continuous spectral representation of dipole form. The transverse cross-section sigma_T for gamma* p -> V p behaves asymptotically as 1/Q^4, while R_V = sigma_L/sigma_T becomes asymptotically constant. Contributions violating s-channel helicity conservation stay at the 10-15% level established in low-energy photoproduction and diffractive hadron--hadron interactions. The data for phi- and rho-meson production for 0 \lsim Q^2 \lsim 20 GeV^2 from HERA are found to be in agreement with these predictions.
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 < 550GeV [800GeV] at 95% C.L. based on the input of swbar^2(LEP+SLD)=0.23165+-0.00024 [swbar^2(LEP)=0.23200 +-0.00027]. With respect to LEP2, it is emphasized that first direct experimental evidence for non-zero non-Abelian couplings among the vector bosons can be obtained even with restricted e+e- luminosity.
We reemphasize the importance of discriminating fermion-loop and bosonic electroweak corrections in the analysis of electroweak precision data. Most recent data are indeed precise enough to require corrections beyond (trivial) fermion loops. An analysis of these data in terms of the observables $Δx \equiv ε_{N1} - ε_{N2}$, $Δy \equiv - ε_{N2}$ and $ε\equiv - ε_{N3}$ identifies the required additional corrections as vertex corrections at the $W^\pm f\bar f^\prime$ and $Z^0 f\bar f$ vertices. Standard-model values for these corrections are consistent with the experimental data.
We examine dimension-six extensions of the standard electroweak Lagrangian which are invariant under local \suu -transformations. The dimension-four trilinear and quadrilinear effective interactions of the vector bosons with one another are found to coincide with the vector boson interactions previously derived from global SU(2) weak isospin symmetry broken by electromagnetism. Supplementing the model by a well-known dimension-six single-parameter quadrupole interaction leads to the most general vector boson self-couplings that can be obtained by addition of dimension-six terms to the standard Lagrangian. We examine in some detail another SU(2)xU(1)-symmetric interaction which contains W_3-B mixing and modifies both vector boson self-couplings and fermionic interactions. Independently of being strongly constrained by the LEP 1 data, the addition of this interaction to the above-mentioned non-standard ones does not change the form of the trilinear and quadrilinear non-standard self-couplings of the vector bosons. Therefore, while being interesting in itself with respect to LEP~1 physics, this term is irrelevant with respect to the phenomenology of the vector-boson self-interactions.