339 research outputs found
A few comments on the high-energy behavior of string scattering amplitudes in warped spacetimes
The high-energy behavior of string scattering in warped spacetimes is studied
to all orders in perturbation theory. If one assumes that the theory is finite,
the amplitudes exactly fall as powers of momentum.Comment: 6 page
R-Current DIS on a Shock Wave: Beyond the Eikonal Approximation
We find the DIS structure functions at strong coupling by calculating
R-current correlators on a finite-size shock wave using AdS/CFT correspondence.
We improve on the existing results in the literature by going beyond the
eikonal approximation for the two lowest orders in graviton exchanges. We argue
that since the eikonal approximation at strong coupling resums integer powers
of 1/x (with x the Bjorken-x variable), the non-eikonal corrections bringing in
positive integer powers of x can not be neglected in the small-x limit, as the
non-eikonal order-x correction to the (n+1)st term in the eikonal series is of
the same order in x as the nth eikonal term in that series. We demonstrate
that, in qualitative agreement with the earlier DIS analysis based on
calculation of the expectation value of the Wilson loop in the shock wave
background using AdS/CFT, after inclusion of non-eikonal corrections DIS
structure functions are described by two momentum scales: Q_1^2 ~ \Lambda^2 \,
A^{1/3}/x and Q_2^2 ~ \Lambda^2 \, A^{2/3}, where \Lambda is the typical
transverse momentum in the shock wave and A is the atomic number if the shock
wave represents a nucleus. We discuss possible physical meanings of the scales
Q_1 and Q_2.Comment: 44 pages, 3 figures; v2: typos corrected, refs added, discussion
extende
Comparing AdS/CFT Calculations to HERA F_2 Data
We show that HERA data for the inclusive structure function F_2(x,Q^2) at
small Bjorken-x and Q^2 can be reasonably well described by a color-dipole
model with an AdS/CFT-inspired dipole-proton cross section. The model contains
only three free parameters fitted to data. In our AdS/CFT-based
parameterization the saturation scale varies in the range of 1-3 GeV becoming
independent of energy/Bjorken-x at very small x. This leads to the prediction
of x-independence of the F_2 and F_L structure functions at very small x. We
provide predictions for F_2 and F_L in the kinematic regions of future
experiments. We discuss the limitations of our approach and its applicability
region, and argue that our AdS/CFT-based model of non-perturbative physics
could be viewed as complimentary to the perturbative description of data based
on saturation/Color Glass Condensate physics.Comment: 23 pages, 10 figures; v3: new plots added showing our model
predictions for charm and longitudinal structure functions and
photoproduction cross-section, discussion extended. The version to appear in
PR
Gluon Structure Function of a Color Dipole in the Light-Cone Limit of Lattice QCD
We calculate the gluon structure function of a color dipole in
near-light-cone SU(2) lattice QCD as a function of . The quark and
antiquark are external non-dynamical degrees of freedom which act as sources of
the gluon string configuration defining the dipole. We compute the color dipole
matrix element of transversal chromo-electric and chromo-magnetic field
operators separated along a direction close to the light cone, the Fourier
transform of which is the gluon structure function. As vacuum state in the pure
glue sector, we use a variational ground state of the near-light-cone
Hamiltonian. We derive a recursion relation for the gluon structure function on
the lattice similar to the perturbative DGLAP equation. It depends on the
number of transversal links assembling the Schwinger string of the dipole.
Fixing the mean momentum fraction of the gluons to the "experimental value" in
a proton, we compare our gluon structure function for a dipole state with four
links with the NLO \emph{MRST} 2002 and the \emph{CTEQAB-0} parameterizations
at . Within the systematic uncertainty we find rather
good agreement. We also discuss the low behavior of the gluon structure
function in our model calculation.Comment: 44 pages, 10 figures, to be in accordance with the variant submitted
to Phys. Rev.
From EMC- and Cronin-effects to signals of quark-gluon plasma
The EMC- and Cronin-effects are explained by a unitarized evolution equation,
where the shadowing and antishadowing corrections are dynamically produced by
gluon fusions. For this sake, an alternative form of the GLR-MQ-ZRS equation is
derived. The resulting integrated and unintegrated gluon distributions in
proton and nuclei are used to analyze the contributions of the initial parton
distributions to the nuclear suppression factor in heavy ion collisions. A
simulation of the fractional energy loss is extracted from the RHIC and LHC
data, where the contributions of the nuclear shadowing and antishadowing
effects are considered. We find a rapid crossover from week energy loss to
strong energy loss at a universal critical energy of gluon jet .Comment: 35 pages, 13 figures, to be published in Int. J. Mod. Phys.
RHIC-tested predictions for low- and high- hadron spectra in nearly central Pb+Pb collisions at the LHC
We study the hadron spectra in nearly central + collisions at RHIC and
LHC in a broad transverse momentum range. We cover the low- spectra using
longitudinally boost-invariant hydrodynamics with initial energy and net-baryon
number densities from the perturbative QCD (pQCD)+saturation model. Build-up of
the transverse flow and sensitivity of the spectra to a single decoupling
temperature \Tdec are studied. Comparison with RHIC data at \ssNN=130 and
200 GeV suggests a rather high value \Tdec=150 MeV. The high- spectra
are computed using factorized pQCD cross sections, nuclear parton
distributions, fragmentation functions, and describing partonic energy loss in
the quark-gluon plasma by quenching weights. Overall normalization is fixed on
the basis of p+(p) data and the strength of energy loss is
determined from RHIC Au+Au data. Uncertainties are discussed. With constraints
from RHIC data, we predict the spectra of hadrons in 5 % most central
Pb+Pb collisions at the LHC energy \ssNN=5500 GeV. Due to the closed
framework for primary production, we can also predict the net-baryon number at
midrapidity, as well as the strength of partonic energy losses at the LHC. Both
at the LHC and RHIC, we recognize a rather narrow crossover region in the
spectra, where the hydrodynamic and pQCD fragmentation components become of
equal size. We argue that in this crossover region the two contributions are to
a good approximation mutually independent. In particular, our results suggest a
wider -region of applicability for hydrodynamical models at the LHC than
at RHIC.Comment: 45 pages, 16 eps-figure
On the Spin content of the Nucleon
A QCD sum rule calculation of Balistky and Ji on the spin content of the
nucleon is done with a different approach to the evaluation of the bilocal
contributions and to the extraction of the nucleon pole residues. The result
obtained is much more numerically stable which puts their conclusion that about
half of the nucleon spin is carried by gluons on firmer ground.Comment: 7 pages, two (eps) figure, minor corrections and one figure adde
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