SU(3) Flux Tubes in a Model of the stochastic Vacuum

We calculate the squared gluon field strengths of a heavy q-$\rm \bar{q}$-pair in the model of the stochastic vacuum. We observe that with increasing separation a chromoelectric flux tube is built. The properties of the emerging flux tube are investigated.Comment: 14, epsf, HD-THEP-94-3

Pre-resonant Charmonium-Nucleon Cross Section in the Model of the Stochastic Vacuum

We calculate the nonperturbative charmonium - nucleon cross sections with the model of the stochastic vacuum, which has been succesfully applied in many high energy reactions. We also give a quantitative discussion of pre-resonance formation and medium effects.Comment: Talk given at QCD Euroconference 99, Montpellier, 7-13 July 1999, 4 pages, Late

Nucleon Structure and High Energy Scattering

If the pomeron is generated by a two gluon exchange there is no a priori reason for a drastic suppression of three gluon exchange with negative parity and charge parity. This would lead to an unacceptably large difference between $p\,p$ and $p\, \bar{p}$ scattering. It is shown that a natural suppression of the $C$=$P$=--1 contribution to high energy scattering is given by a cluster structure of the nucleon.Comment: 10 pages, epsf.sty and a4wide.sty, 8 eps figs included complete PS-file (with all figs) available at: http://www.thphys.uni-heidelberg.de/~rueter

The field strength correlator from QCD sum rules

The gauge invariant two-point correlator for the gluon field strength tensor in analysed by means of the QCD sum rule method. To this end, we make use of a relation of this correlator to a two-point function for a quark-gluon hybrid in the limit of the quark mass going to infinity. From the sum rules a relation between the gluon correlation length and the gluon condensate is obtained.Comment: 5 pages, 1 figure, QCD Euroconferenc

QCD sum rule analysis of the field strength correlator

The gauge invariant two-point correlator for the gluon field strength tensor is analysed by means of the QCD sum rule method. To this end, we make use of a relation of this correlator to a two-point function for a quark-gluon hybrid in the limit of the quark mass going to infinity. From the sum rules a relation between the gluon correlation length and the gluon condensate is obtained. We briefly compare our results to recent determinations of the field strength correlator on the lattice.Comment: 16 pages, 1 figur

Nonperturbative and perturbative aspects of photo- and electroproduction of vector mesons

We discuss various aspects of vector meson production, first analysing the interplay between perturbative and nonperturbative aspects of the QCD calculation. Using a general method adapted to incorporate both perturbative and nonpertubative aspects, we show that nonperturbative effects are important for all experimentally available values of the photon virtuality Q2. We compare the huge amount of experimental information now available with our theoretical results obtained using a specific nonperturbative model without free parameters, showing that quite simple features are able to explain the data.Comment: 19 page

Energy Losses (Gains) of Massive Coloured Particles in Stochastic Colour Medium

The propagation of massive coloured particles in stochastic background chromoelectric field is studied using the semiclassical equations of motion. Depending on the nature of the stochastic background we obtain the formulae for the energy losses of heavy coloured projectile in nonperturbative hadronic medium and for the energy gains in the stochastic field present, e.g., in the turbulent plasma. The result appears to be significantly dependent on the form of the correlation function of stochastic external fieldComment: 9 pages, BI-TP 94/15, plain LaTe

Two Photon Reactions at High Energies

Cross sections for the reactions gamma^(*) gamma^(*) --> hadrons and gamma^(*) gamma^(*) --> 2 vector mesons are calculated as functions of energy (sqrt(s) > 20 GeV) and photon virtualities. Good agreement with experiment is obtained for the total hadronic cross section and, after allowing for a valence-quark contribution from the hadronic part of the photon, with the photon structure function at small x. The cross section for vector meson production are shown to be experimentally accessible for moderate values of Q^2. This is sufficient to probe the nature of the hard pomeron which has recently been proposed.Comment: some changes in style, physics unchanged, final version to appear in Phys.Rev.D, LaTeX2e, graphicx package, 15 eps-figures, 15p

The Semiclassical Gluon Distribution at Next-to-Leading Order

The interaction of the partonic fluctuation of the virtual photon in deep inelastic scattering with soft color fields describing the hadron is treated in an eikonal approximation. It is known that, in this approach, the small-x limit of the leading-order gluon distribution xg(x,Q^2) is a constant characterizing the averaged local field strength in the target. Matching the next-to-leading order calculation in this semiclassical framework with the one-loop parton model result, we obtain the next-to-leading order contribution to xg(x,Q^2). It shows a ln(1/x) enhancement at small x and is sensitive to the large distance structure of the target. The final expression is a simple integral over non-Abelian eikonal factors measuring the target color field. We derive a quantitative relation between the short-distance cutoff of this integral and the scale of the gluon distribution function in the MS-bar scheme. Our calculation demonstrates that higher order contributions can be systematically included in the semiclassical approach.Comment: 19 pages LaTeX, 8 figures include

Diffractive Exclusive Photon Production in Deep Inelastic Scattering

Predictions for deep Virtual Compton Scattering are obtained in a two-component dipole model of diffraction. The model automatically includes hard and soft components and implicitly allows for ``hadronic'' contributions via large dipoles. It is also applicable to real Compton Scattering, which provides an important constraint.Comment: 9 pages, 2 figure