650 research outputs found
SU(3) Flux Tubes in a Model of the stochastic Vacuum
We calculate the squared gluon field strengths of a heavy 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
Parton Saturation-An Overview
The idea of partons and the utility of using light-cone gauge in QCD are
introduced. Saturation of quark and gluon distributions are discussed using
simple models and in a more general context. The Golec-Biernat W\usthoff model
and some simple phenomenology are described. A simple, but realistic, equation
for unitary, the Kovchegov equation, is discussed, and an elementary derivation
of the JIMWLK equation is given.Comment: Cargese Lectures, 34 pages, 19 figure
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
Decomposition of the QCD String into Dipoles and Unintegrated Gluon Distributions
We present the perturbative and non-perturbative QCD structure of the
dipole-dipole scattering amplitude in momentum space. The perturbative
contribution is described by two-gluon exchange and the non-perturbative
contribution by the stochastic vacuum model which leads to confinement of the
quark and antiquark in the dipole via a string of color fields. This QCD string
gives important non-perturbative contributions to high-energy reactions. A new
structure different from the perturbative dipole factors is found in the
string-string scattering amplitude. The string can be represented as an
integral over stringless dipoles with a given dipole number density. This
decomposition of the QCD string into dipoles allows us to calculate the
unintegrated gluon distribution of hadrons and photons from the dipole-hadron
and dipole-photon cross section via kT-factorization.Comment: 43 pages, 14 figure
Elastic and quasi-elastic and scattering in the Dipole Model
We have in earlier papers presented an extension of Mueller's dipole cascade
model, which includes sub-leading effects from energy conservation and running
coupling as well as colour suppressed saturation effects from pomeron loops via
a ``dipole swing''. The model was applied to describe the total and diffractive
cross sections in and collisions, and also the elastic cross
section in scattering.
In this paper we extend the model to describe the corresponding quasi-elastic
cross sections in , namely the exclusive production of vector mesons
and deeply virtual compton scattering. Also for these reactions we find a good
agrement with measured cross sections. In addition we obtain a reasonable
description of the -dependence of the elastic and quasi-elastic
cross sections
A draft genome sequence of Nicotiana benthamiana to enhance molecular plant-microbe biology research
Nicotiana benthamiana is a widely used model plant species for the study of fundamental questions in molecular plant-microbe interactions and other areas of plant biology. This popularity derives from its well-characterized susceptibility to diverse pathogens and, especially, its amenability to virus-induced gene silencing and transient protein expression methods. Here, we report the generation of a 63-fold coverage draft genome sequence of N. benthamiana and its availability on the Sol Genomics Network for both BLAST searches and for downloading to local servers. The estimated genome size of N. benthamiana is 3 Gb (gigabases). The current assembly consists of approximately 141,000 scaffolds, spanning 2.6 Gb with 50% of the genome sequence contained within scaffolds >89 kilobases. Of the approximately 16,000 N. benthamiana unigenes available in GenBank, >90% are represented in the assembly. The usefulness of the sequence was demonstrated by the retrieval of N. benthamiana orthologs for 24 immunity-associated genes from other species including Ago2, Ago7, Bak1, Bik1, Crt1, Fls2, Pto, Prf, Rar1, and mitogen-activated protein kinases. The sequence will also be useful for comparative genomics in the Solanaceae family as shown here by the discovery of microsynteny between N. benthamiana and tomato in the region encompassing the Pto and Prf genes
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