1,061 research outputs found
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
- …