2,480 research outputs found
Vector potential versus colour charge density in low-x evolution
We reconsider the evolution equations for multigluon correlators derived in
hep-ph/9709432. We show how to derive these equations directly in terms of
vector potentials (or colour field strength) avoiding the introduction of the
concept of colour charge density in the intermediate steps. The two step
procedure of deriving the evolution of the charge density correlators followed
by the solution of classical equations for the vector potentials is shown to be
consistent with direct derivation of evolution for vector potentials. In the
process we correct some computational errors of hep-ph/9709432 and present the
corrected evolution equations which have a somewhat simpler appearance.Comment: 15 pages, 1 figure, changes made referee report, to be published in
Phys. Rev
Shadowing of gluons in perturbative QCD: A comparison of different models
We investigate the different perturbative QCD-based models for nuclear
shadowing of gluons. We show that in the kinematic region appropriate to RHIC
experiment, all models give similar estimates for the magnitude of gluon
shadowing. At scales relevant to LHC, there is a sizable difference between
predictions of the different models.Comment: 11 pages, 4 figure
Dilepton production from the Color Glass Condensate
We consider dilepton production in high energy proton-nucleus (and very
forward nucleus-nucleus) collisions. Treating the target nucleus as a Color
Glass Condensate and describing the projectile proton (nucleus) as a collection
of quarks and gluons as in the parton model, we calculate the differential
cross section for dilepton production in quark-nucleus scattering and show that
it is very sensitive to the saturation scale characterizing the target nucleus.Comment: 9 pages LaTeX document, 1 postscript figur
Prompt photons at RHIC
We calculate the inclusive cross section for prompt photon production in
heavy-ion collisions at RHIC energies ( GeV and
GeV) in the central rapidity region including next-to-leading order,
, radiative corrections, initial state nuclear
shadowing and parton energy loss effects. We show that there is a significant
suppression of the nuclear cross section, up to at
GeV, due to shadowing and medium induced parton energy loss effects. We find
that the next-to-leading order contributions are large and have a strong
dependence.Comment: 9 pages, 5 figures, expanded discussion of the K facto
Dilepton low suppression as an evidence of the Color Glass Condensate
The dilepton production is investigated in proton-nucleus collisions in the
forward region using the Color Glass Condensate approach. The transverse
momentum distribution (), more precisely the low region, where the
saturation effects are expected to increase, is analyzed. The ratio between
proton-nucleus and proton-proton differential cross section for RHIC and LHC
energies is evaluated, showing the effects of saturation at small , and
presenting a Cronin type peak at moderate . These features indicate the
dilepton as a most suitable probe to study the properties of the saturated
regime and the Cronin effect.Comment: 10 pages, 8 figures, replaced with the version to appear in Physical
Review
Random walks of partons in SU(N_c) and classical representations of color charges in QCD at small x
The effective action for wee partons in large nuclei includes a sum over
static color sources distributed in a wide range of representations of the
SU(N_c) color group. The problem can be formulated as a random walk of partons
in the N_c-1 dimensional space spanned by the Casimirs of SU(N_c). For a large
number of sources, k >> 1, we show explicitly that the most likely
representation is a classical representation of order O(\sqrt{k}). The quantum
sum over representations is well approximated by a path integral over classical
sources with an exponential weight whose argument is the quadratic Casimir
operator of the group. The contributions of the higher N_c-2 Casimir operators
are suppressed by powers of k. Other applications of the techniques developed
here are discussed briefly.Comment: 51 pages, includes 3 eps file
JIMWLK evolution in the Gaussian approximation
We demonstrate that the Balitsky-JIMWLK equations describing the high-energy
evolution of the n-point functions of the Wilson lines (the QCD scattering
amplitudes in the eikonal approximation) admit a controlled mean field
approximation of the Gaussian type, for any value of the number of colors Nc.
This approximation is strictly correct in the weak scattering regime at
relatively large transverse momenta, where it reproduces the BFKL dynamics, and
in the strong scattering regime deeply at saturation, where it properly
describes the evolution of the scattering amplitudes towards the respective
black disk limits. The approximation scheme is fully specified by giving the
2-point function (the S-matrix for a color dipole), which in turn can be
related to the solution to the Balitsky-Kovchegov equation, including at finite
Nc. Any higher n-point function with n greater than or equal to 4 can be
computed in terms of the dipole S-matrix by solving a closed system of
evolution equations (a simplified version of the respective Balitsky-JIMWLK
equations) which are local in the transverse coordinates. For simple
configurations of the projectile in the transverse plane, our new results for
the 4-point and the 6-point functions coincide with the high-energy
extrapolations of the respective results in the McLerran-Venugopalan model. One
cornerstone of our construction is a symmetry property of the JIMWLK evolution,
that we notice here for the first time: the fact that, with increasing energy,
a hadron is expanding its longitudinal support symmetrically around the
light-cone. This corresponds to invariance under time reversal for the
scattering amplitudes.Comment: v2: 45 pages, 4 figures, various corrections, section 4.4 updated, to
appear in JHE
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
QCD at small x and nucleus-nucleus collisions
At large collision energy sqrt(s) and relatively low momentum transfer Q, one
expects a new regime of Quantum Chromo-Dynamics (QCD) known as "saturation".
This kinematical range is characterized by a very large occupation number for
gluons inside hadrons and nuclei; this is the region where higher twist
contributions are as large as the leading twist contributions incorporated in
collinear factorization. In this talk, I discuss the onset of and dynamics in
the saturation regime, some of its experimental signatures, and its
implications for the early stages of Heavy Ion Collisions.Comment: Plenary talk given at QM2006, Shanghai, November 2006. 8 pages, 8
figure
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