The Missing Odderon

In contrast to theoretical expectations, experimental results at sqrt(s)=200 GeV for the reaction gamma p --> pi0 X show no evidence for odderon exchange. The upper limit on the cross section is an order of magnitude smaller than the theoretical estimate. It is argued that chiral symmetry leads to a large suppression, taking the thoeretical estimates well below the data. Two additional arguments are presented which may decrease the theoretical estimate further. The calculations are more sensitive to the assumptions made in evaluating the hadronic scattering amplitude than in the processes considered previously and lattice gauge calculations indicate that the odderon intercept may be appreciably lower than usually assumed. These two latter effects are particularly relevant for the reactions gamma p --> f2(1270)X and gamma p --> a2(1320)X for which the data upper limits are also below the theoretical predictions, but not so dramatically as for gamma p --> pi0 X.Comment: 12 pages, 3 figure

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

Field strength correlators in QCD with dynamical fermions

We determine, by numerical simulations on a lattice, the gauge-invariant two-point correlation functions of the gauge field strengths in the QCD vacuum with four flavours of dynamical staggered fermions.Comment: Talk given at the ``XVth International Symposium on Lattice Field Theory'', Edinburgh (UK), July 22nd-26th 1997 (LATTICE 97); 3 pages, LaTeX file, uses ``espcrc2.sty''+ 2 PS figure

Universality in the Electroproduction of Vector Mesons

We study universality in the electroproduction of vector mesons using a unified nonperturbative approach which has already proved to reproduce extremely well the available experimental data. In this framework, after the extraction of factors that are specific of each vector meson, we arrive at a reduced integrated elastic cross section which is universal. Our calculations suggest a finite infrared behavior for the strong coupling constant.Comment: 22 pages, 10 figure

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

Log(1/x) Gluon Distribution and Structure Functions in the Loop-Loop Correlation Model

We consider the interaction of the partonic fluctuation of a scalar ``photon'' with an external color field to calculate the leading and next-to-leading order gluon distribution of the proton following the work done by Dosch-Hebecker-Metz-Pirner. We relate these gluon distributions to the short and long distance behavior of the cross section of an adjoint dipole scattering off a proton. The leading order result is a constant while the next-to-leading order result shows a ln(1/x) enhancement at small x. To get numerical results for the gluon distributions at the initial scale Q^2_0=1.8 GeV^2, we compute the adjoint dipole-proton cross section in the loop-loop correlation model. Quark distributions at the same initial scale are parametrized according to Regge theory. We evolve quark and gluon distributions to higher Q^2 values using the DGLAP equation and compute charm and proton structure functions in the small-x region for different Q^2 values.Comment: 13 pages, 10 figures,revised version,references added, typos corrected, to be published in Eur. Phys. Journal