50 research outputs found
Quantum structure of the non-Abelian Weizsacker-Williams field for a very large nucleus
We consider the McLerran-Venugopalan model for calculation of the small-
part of the gluon distribution function for a very large ultrarelativistic
nucleus at weak coupling. We construct the Feynman diagrams which correspond to
the classical Weizs\"{a}cker-Williams field found previously [Yu. V. Kovchegov,
Phys. Rev. D 54, 5463 (1996)] as a solution of the classical equations of
motion for the gluon field in the light-cone gauge. Analyzing these diagrams we
obtain a limit for the McLerran-Venugopalan model. We show that as long as this
limit is not violated a classical field can be used for calculation of
scattering amplitudes.Comment: 13 pages, REVTeX, 9 figure
Unitarization of the BFKL Pomeron on a Nucleus
We analyze the evolution equation describing all multiple hard pomeron
exchanges in a hadronic or nuclear structure functions that was proposed
earlier. We construct a perturbation series providing us with an exact solution
to the equation outside of the saturation region. The series demonstrates how
at moderately high energies the corrections to the single BFKL pomeron exchange
contribution which are due to the multiple pomeron exchanges start unitarizing
total deep inelastic scattering cross section. We show that as energy increases
the scattering cross section of the quark-antiquark pair of a fixed transverse
separation on a hadron or nucleus given by the solution of our equation inside
of the saturation region unitarizes and becomes independent of energy. The
corresponding F_2 structure function also unitarizes and becomes linearly
proportional to ln s. We also discuss possible applications of the developed
technique to diffraction.Comment: REVTeX, 20 pages, 6 figure
Eikonal Evolution and Gluon Radiation
We give a simple quantum mechanical formulation of the eikonal propagation
approximation, which has been heavily used in recent years in problems
involving hadronic interactions at high energy. This provides a unified
framework for several approaches existing in the literature. We illustrate this
scheme by calculating the total, elastic, inelastic and diffractive DIS cross
sections, as well as gluon production in high energy hadronic collisions. From
the q-qbar-g-component of the DIS cross sections, we straightforwardly derive
low x evolution equations for inelastic and diffractive DIS distribution
functions. In all calculations, we provide all order 1/N corrections to the
results existing in the literature.Comment: 40 pages, LaTeX, 3 eps-figures, typos corrected, to be published in
PR
Gluon Radiation and Coherent States in Ultrarelativistic Nuclear Collisions
We explore the correspondence between classical gluon radiation and quantum
radiation in a coherent state for gluons produced in ultrarelativistic nuclear
collisions. The expectation value of the invariant momentum distribution of
gluons in the coherent state is found to agree with the gluon number
distribution obtained classically from the solution of the Yang-Mills
equations. A criterion for the applicability of the coherent state formalism to
the problem of radiation in ultrarelativistic nucleus-nucleus collisions is
discussed. This criterion is found to be fulfilled for midrapidity gluons with
perturbative transverse momenta larger than about 1-2 GeV and produced in
collisions between valence partons.Comment: 15 pages, 6 figures, RevTeX (with epsf, psfig style files
The initial energy density of gluons produced in very high energy nuclear collisions
In very high energy nuclear collisions, the initial energy of produced gluons
per unit area per unit rapidity, , is equal to , where is proportional to the gluon density per unit
area of the colliding nuclei. For an SU(2) gauge theory, we perform a
non--perturbative numerical computation of the function . It
decreases rapidly for small but varies only by %, from
to , for a wide range 35.36--296.98 in , including the range relevant for collisions at RHIC and LHC. Extrapolating
to SU(3), we estimate the initial energy per unit rapidity for Au-Au collisions
in the central region at RHIC and LHC.Comment: 11 pages, Latex, 3 figures; revised version-includes additional
numerical data; reference adde
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
Diffractive Structure Function in a Quasi-Classical Approximation
We derive an expression for diffractive F_2 structure function which should
be valid at small x for quasi-elastic scattering on a hadron and for
quasi-elastic scattering on a large nucleus. This expression includes multiple
rescatterings of the quark-antiquark pair produced by the virtual photon off
the sources of color charge in a quasi-classical approximation. We find that
there is a relation between such diffractive production and inclusive
processes. In the former, one averages over all colors of sources before
squaring the amplitude, and in the latter one first squares the amplitude and
then averages it in the hadron's or nuclear wave function. We show that in the
limit of a large virtuality of the photon Q^2 the diffractive structure
function becomes linearly proportional to the gluon distribution of the hadron
or nucleus, therefore proving that in this sense diffraction is a leading twist
effect.Comment: 11 pages, 4 figures, REVTeX, some changes included, a mistake is
correcte
Has HERA reached a new QCD regime?
These notes are a summary of our efforts to answer the question in the title.
Our answer is in the affirmative as: (i) HERA data indicate a large value of
the gluon structure function; (ii) no contradictions with the asymptotic
predictions of high density QCD have been observed; and (iii) the numerical
estimates of our model give a natural description of the size of deviation from
the routine DGLAP explanation. We discuss the alternative approaches and
possible new experiments.Comment: 29 pages, 37 figures in eps file