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

Inelastic diffractive proton-proton scattering in nonperturbative QCD

We examine diffractive proton-proton scattering. Using a functional integral approach we derive the scattering amplitudes, which are governed by the expectation value of lightlike Wegner-Wilson loops. This expectation value is then evaluated using a cumulant expansion and the model of the stochastic vacuum. From the scattering amplitudes we calculate total and differential cross sections for high centre of mass energy and small momentum transfer and compare with experiments.Comment: 4 pages, 2 figures, to appear in the Proceedings of the International Workshop on Diffraction 2000, Cetraro, Italy, 2-7 Sept. 200

High-energy exclusive leptoproduction of vector mesons

The physics of diffractive vector meson production in virtual photon nucleon scattering at NMC energies is reviewed. A particular attention is paid to the physical aspects of the reaction and how they influence the observables. The reaction is a good probe to investigate both soft exchange mechanisms and hadronic wave functions. Extension to either HERA or ELFE kinematics is sketched out.Comment: Contribution to the workshop "ELFE at DESY" (St-Malo, Sep. 96), 15 pages, LaTeX file, 8 figures, uses eps

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

Description of elastic vectormeson production and F_2 by two pomerons

Using the Model of the Stochastic Vacuum many diffractive processes have been calculated by investigating the dipole-dipole scattering at a cm-energy of 20 GeV. In this work we extend the calculation to larger energies and small dipoles. We assume that there are two pomerons, the hard- and the soft-pomeron, which cause the different energy dependence for processes dominated by small or large dipoles. The physical processes are obtained by smearing the dipole-dipole amplitude with wavefunctions. For small dipoles the leading perturbative contribution is taken into account. By that way we can describe in addition to the already calculated low energy results (20 GeV) also the HERA data for the considered processes in nearly the whole energy and Q^2 range.Comment: LaTeX2e, 4pp, espcrc2mod.sty (appended, espcrc2 with corrected error), graphicx.sty, 17 eps-figures. Talk presented at QCD98, Montpellier, France (Nucl.Phys.B Proc.Suppl.

High-energy elementary amplitudes from quenched and full QCD

Making use of the Stochastic Vacuum Model and the gluon gauge-invariant two-point correlation function, determined by numerical simulation on the lattice in both quenched approximation and full QCD, we calculate the elementary (quark-quark) scattering amplitudes in the momentum transfer space and at asymptotic energies. Our main conclusions are the following: (1) the amplitudes decrease monotonically as the momentum transfer increases; (2) the decreasing is faster when going from quenched approximation to full QCD (with decreasing quark masses) and this effect is associated with the increase of the correlation lengths; (3) dynamical fermions generate two components in the amplitude at small momentum transfer and the transition between them occurs at momentum transfer near 1 GeV$^2$. We also obtain analytical parametrizations for the elementary amplitudes, that are suitable for phenomenological uses, and discuss the effects of extrapolations from the physical regions investigated in the lattice.Comment: 6 pages, 4 figures, aps-revtex4, to be published in Physics Letters

Exact relations for thermodynamics of heavy quarks

We derive finite-temperature sum rules for excesses in internal energy and in (volume-integrated) pressure arising due to presence of heavy quarks in SU(N) gluon plasma. In the limit of zero temperature our formulae reduce to the Michael-Rothe sum rules. The excesses in energy and pressure of the gluon plasma are related to expectation values of certain gluon condensates, and, simultaneously, to the heavy quark potential. The sum rules lead to a known relation between the internal energy and the potential, and to a new expression for the excess in the pressure. The pressure appears in the free energy as a generalized force associated with variations of the spatial size of the heavy-quark system. We find that the excess in gluonic pressure around a heavy quarkonium is always negative. Finally, we derive an exact equation of state that provides a relationship between the gluonic energy and pressure of heavy quarks.Comment: 7 page

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