786 research outputs found
Anomalous dimensions from rotating open strings in AdS/CFT
We propose a new entry within the dictionary of the AdS/CFT duality at strong
coupling: in the limit of a large spin or a large R-charge, the anomalous
dimension of the gauge theory operator dual to a semiclassical rotating string
is proportional to the string proper length. This conjecture is motivated by a
generalization to strings of the rule for computing anomalous dimensions of
massive particles and supergravity fields in the anti-de Sitter space. We show
that this proportionality holds for a rotating closed string in global AdS
space, representing a high spin operator made of fields in the adjoint
representation. It is also valid for closed strings rotating in
(representing operators with large R-charge), for closed strings with multiple
AdS spin, and for giant magnons. Based on this conjecture, we calculate the
anomalous dimension of operators made of fields in the fundamental
representation, associated with high spin mesons, and which are represented by
rotating open strings attached to probe D7-branes. The result is a logarithmic
dependence upon the spin, , similar to the
closed string case. We show that the operator properties --- anomalous
dimension and spin --- are obtained from measurements made by a local observer
in the anti-de Sitter space. For the open string case, this ensures that these
quantities are independent of the mass scale introduced by the D7-branes (the
quark mass), as expected on physical grounds. In contrast, properties of the
gauge theory states, like the energy, correspond to measurements by a gauge
theory observer and depend upon the mass scale --- once again, as expected.Comment: V2: two related references include
Universality of the saturation scale and the initial eccentricity in heavy ion collisions
Recent estimates that Color Glass Condensate initial conditions may generate
a larger initial eccentricity for noncentral relativistic heavy ion collisions
(relative to the initial eccentricity assumed in earlier hydrodynamic
calculations) have raised the possibility of a higher bound on the viscosity of
the Quark Gluon Plasma. We show that this large initial eccentricity results in
part from a definition of the saturation scale as proportional to the number of
nucleons participating in the collision. A saturation scale proportional to the
nuclear thickness function (and therefore independent of the probe) leads to a
smaller eccentricity, albeit still larger than the value used in hydrodynamic
models. Our results suggest that the early elliptic flow in heavy ion
collisions (unlike multiplicity distributions) is sensitive to the universality
of the saturation scale in high energy QCD.Comment: 5 pages, 3 figures, RevTE
Low x saturation at HERA ?
We compare the predictions of two distinct dipole models for inclusive and
exclusive diffractive processes. While only one of these dipole models contains
perturbative saturation dynamics, we show that the predictions of both models
are fully consistent with the available HERA data, indicating no compelling
evidence for saturation at present HERA energies.Comment: 8 pages, 4 figures, presented at the 26th
Montreal-Rochester-Syracuse-Toronto (MRST) conference held at Concordia
University, Montreal, Canada, 12th-14th May 2004. To appear in the
proceeding
Infrared instability from nonlinear QCD evolution
Using the Balitsky-Kovchegov (BK) equation as an explicit example, we show
that nonlinear QCD evolution leads to an instability in the propagation toward
the infrared of the gluon transverse momentum distribution, if one starts with
a state with an infrared cut-off. This effect takes the mathematical form of
rapidly moving traveling wave solutions of the BK equation, which we
investigate by numerical simulations. These traveling wave solutions are
different from those governing the transition to saturation, which propagate
towards the ultraviolet. The infrared wave speed, formally infinite for the
leading order QCD kernel, is determined by higher order corrections. This
mechanism could play a role in the rapid decrease of the mean free path in the
Color Glass Condensate scenario for heavy ion collisions.Comment: 12 pages, 8 figures. Update to match version in Nuclear Physics
From RHIC to EIC: Nuclear Structure Functions
We study the nuclear structure function and its logarithmic
derivative in the high energy limit (small region) using the Color Glass
Condensate formalism. In this limit the structure function depends on the
quark anti-quark dipole-target scattering cross section . The same dipole cross section appears in single hadron and hadron-photon
production cross sections in the forward rapidity region in deuteron
(proton)-nucleus collisions at high energy, i.e. at RHIC and LHC. We use a
parameterization of the dipole cross section, which has successfully been used
to describe the deuteron-gold data at RHIC, to compute the nuclear structure
function and its log derivative (which is related to gluon
distribution function in the double log limit). We provide a quantitative
estimate of the nuclear shadowing of and the gluon distribution
function in the kinematic region relevant to a future Electron-Ion Collider.Comment: 13 pages, 6 figure
Violation of kT factorization in quark production from the Color Glass Condensate
We examine the violation of the kT factorization approximation for quark
production in high energy proton-nucleus collisions. We comment on its
implications for the open charm and quarkonium production in collider
experiments.Comment: 4 pages, 6 figures, contribution to proceedings of Quark Matter 2005,
Budapest, Aug 4-
Determination of nuclear parton distribution functions and their uncertainties at next-to-leading order
Nuclear parton distribution functions (NPDFs) are determined by global
analyses of experimental data on structure-function ratios F_2^A/F_2^{A'} and
Drell-Yan cross-section ratios \sigma_{DY}^A/\sigma_{DY}^{A'}. The analyses are
done in the leading order (LO) and next-to-leading order (NLO) of running
coupling constant \alpha_s. Uncertainties of the NPDFs are estimated in both LO
and NLO for finding possible NLO improvement. Valence-quark distributions are
well determined, and antiquark distributions are also determined at x<0.1.
However, the antiquark distributions have large uncertainties at x>0.2. Gluon
modifications cannot be fixed at this stage. Although the advantage of the NLO
analysis, in comparison with the LO one, is generally the sensitivity to the
gluon distributions, gluon uncertainties are almost the same in the LO and NLO.
It is because current scaling-violation data are not accurate enough to
determine precise nuclear gluon distributions. Modifications of the PDFs in the
deuteron are also discussed by including data on the proton-deuteron ratio
F_2^D/F_2^p in the analysis. A code is provided for calculating the NPDFs and
their uncertainties at given x and Q^2 in the LO and NLO.Comment: 15 pages, LaTeX, 22 eps files, to appear in PRC. A code for
calculating our nuclear parton distribution functions and their uncertainties
can be obtained from http://research.kek.jp/people/kumanos/nuclp.htm
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
QCD traveling waves at non-asymptotic energies
Using consistent truncations of the BFKL kernel, we derive analytical
traveling-wave solutions of the Balitsky-Kovchegov saturation equation for both
fixed and running coupling. A universal parametrization of the ``interior'' of
the wave front is obtained and compares well with numerical simulations of the
original Balitsky-Kovchegov equation, even at non-asymptotic energies. Using
this universal parametrization, we find evidence for a traveling-wave pattern
of the dipole amplitude determined from the gluon distribution extracted from
deep inelastic scattering data.Comment: 10 pages, 5 figures, minor revision, version to appear in PL
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