110 research outputs found
A Property of Recombination in Polarized Hadronic Targets
The triple gluon-ladder vertex is shown to project the outgoing gluon in
either polarization state with equal probability up to the leading
double-ln(x)ln() approximation. This implies that the -evolution of
is free from recombination effects to this level of
approximation.Comment: latex, 6 pages AZPH-TH/92-29 Phys. Rev D 50 in press 199
QCD Predictions for the Transverse Energy Flow in Deep-Inelastic Scattering in the Small x HERA Regime
The distribution of transverse energy, , which accompanies
deep-inelastic electron-proton scattering at small , is predicted in the
central region away from the current jet and proton remnants. We use BFKL
dynamics, which arises from the summation of multiple gluon emissions at small
, to derive an analytic expression for the flow. One interesting
feature is an increase of the distribution with
decreasing , where . We perform a
numerical study to examine the possibility of using characteristics of the
distribution as a means of identifying BFKL dynamics at HERA.Comment: 16 pages, REVTEX 3.0, no figures. (Hardcopies of figures available on
request from Professor A.D. Martin, Department of Physics, University of
Durham, DH1 3LE, England.) Durham preprint : DTP/94/0
Elliptic flow at collider energies and cascade string models: The role of hard processes and multi-Pomeron exchanges
Centrality, rapidity, and transverse momentum dependence of hadron elliptic
flow is studied in Au+Au collisions at BNL RHIC energies within the microscopic
quark-gluon string model. The QGSM predictions coincide well with the
experimental data at AGeV. Further investigations reveal that
multi-Pomeron exchanges and hard gluon-gluon scattering in primary collisions,
accompanied by the rescattering of hadrons in spatially anisotropic system, are
the key processes needed for an adequate description of the data. These
processes become essentially important for heavy-ion collisions at full RHIC
energy AGeV.Comment: LATEX, 12 pages incl. 4 figures, to be published in Phys. Lett.
Generalized parton distributions and Deeply Virtual Compton Scattering in Color Glass Condensate model
Within the framework of the Color Glass Condensate model, we evaluate quark
and gluon Generalized Parton Distributions (GPDs) and the cross section of
Deeply Virtual Compton Scattering (DVCS) in the small- region. We
demonstrate that the DVCS cross section becomes independent of energy in the
limit of very small , which clearly indicates saturation of the DVCS
cross section. Our predictions for the GPDs and the DVCS cross section at
high-energies can be tested at the future Electron-Ion Collider and in
ultra-peripheral nucleus-nucleus collisions at the LHC.Comment: 20 pages, 8 Figure
Geometric Scaling in a Symmetric Saturation Model
We illustrate geometric scaling for the photon-proton cross section with a
very simple saturation model. We describe the proton structure function F2 at
small x in a wide kinematical range with an elementary functional form and a
small number of free parameters. We speculate that the symmetry between low and
high Q2 recently discovered in the data could be related to a well-known
symmetry of the two-gluon- exchange dipole-dipole cross section.Comment: 15 pages, 4 figure
Exact resolution of the Baxter equation for reggeized gluon interactions
The interaction of reggeized gluons in multi-colour QCD is considered in the
Baxter-Sklyanin representation, where the wave function is expressed as a
product of Baxter functions Q(lambda) and a pseudo-vacuum state. We find n
solutions of the Baxter equation for a composite state of n gluons with poles
of rank r in the upper lambda semi-plane and of rank n-1-r in the lower lambda
semi-plane (0 leq r leq n-1). These solutions are related by n-2 linear
equations with coefficients depending on coth (pi lambda). The poles cancel in
the wave function, bilinear combination of holomorphic and anti-holomorphic
Baxter functions, guaranteeing its normalizability. The quantization of the
intercepts of the corresponding Regge singularities appears as a result of the
physical requirements that the holomorphic energies for all solutions of the
Baxter equation are the same and the total energies, calculated around two
singularities lambda, lambda^* --> + i or -i, coincide. It results in simple
properties of the zeroes of the Baxter functions. For illustration we calculate
the parameters of the reggeon states constructed from three and four gluons.
For the Odderon the ground state has conformal spin |m -m | = 1 and its
intercept equals unity. The ground state of four reggeized gluons possesses
conformal spin 2 and its intercept turns out to be higher than that for the
BFKL Pomeron. We calculate the anomalous dimensions of the corresponding
operators for arbitrary alpha_s/omega.Comment: LaTex, 42 pages, 8 .ps figures. Expanded and improved versio
Nuclear effects in the Drell-Yan process at very high energies
We study Drell-Yan (DY) dilepton production in proton(deuterium)-nucleus and
in nucleus-nucleus collisions within the light-cone color dipole formalism.
This approach is especially suitable for predicting nuclear effects in the DY
cross section for heavy ion collisions, as it provides the impact parameter
dependence of nuclear shadowing and transverse momentum broadening, quantities
that are not available from the standard parton model. For p(D)+A collisions we
calculate nuclear shadowing and investigate nuclear modification of the DY
transverse momentum distribution at RHIC and LHC for kinematics corresponding
to coherence length much longer than the nuclear size. Calculations are
performed separately for transversely and longitudinally polarized DY photons,
and predictions are presented for the dilepton angular distribution.
Furthermore, we calculate nuclear broadening of the mean transverse momentum
squared of DY dileptons as function of the nuclear mass number and energy. We
also predict nuclear effects for the cross section of the DY process in heavy
ion collisions. We found a substantial nuclear shadowing for valence quarks,
stronger than for the sea.Comment: 46 pages, 18 figures, title changed and some discussion added,
accepted for publication in PR
Photoproduction of J/psi mesons at high energies in parton model and k_t-faktorization approach
We consider J/psi meson photoproduction on protons at high energies at the
leading order in alpha_s using conventional parton model and k_t-factorization
approach of QCD. It is shown that in the both cases the colour singlet
mechanism gives correct description for experimental data from HERA for the
total cross section and for the J/psi meson z-spectrum at realistic values of a
c-quark mass and meson wave function at the origin Psi (0). At the same time
our predictions for p_t-spectrum of J/psi meson and for p_t dependence of the
spin parameter alpha obtained in k_t-factorization approach are very different
from the results obtained in conventional parton model. Such a way the
experimental study of a polarized J/psi meson production at the large p_t
should be a direct test of BFKL gluons.Comment: LaTeX2e, 11 pages plus 8 fig. using epsfig.st
Transition to meson-dominated matter at RHIC. Consequences for kaon flow
Anisotropic flow of kaons and antikaons is studied in heavy-ion collisions at
CERN SPS and BNL RHIC energies within the microscopic quark-gluon string model.
In the midrapidity range the directed flow of kaons v_1 differs considerably
from that of antikaons at SPS energy (E_{lab} = 160 AGeV), while at RHIC energy
(\sqrt{s} = 130 AGeV) the excitation functions of both, kaon and antikaon,
flows coincide within the statistical error bars. The change is attributed to
formation of dense meson-dominated matter at RHIC, where the differences in
interaction cross-sections of kaons and antikaons become unimportant. The time
evolution of the kaon anisotropic flow is also investigated. The elliptic flow
of these hadrons is found to develop at midrapidity at times 3 < t < 10 fm/c,
which is much larger than the nuclear passing time t^{pass} = 0.12 fm/c. As a
function of transverse momentum the elliptic flow increases almost linearly
with rising p_t. It stops to rise at p_t > 1.5 GeV/c reaching the saturation
value .Comment: REVTEX, 14 pages, 4 figure
Nonperturbative Effects in Gluon Radiation and Photoproduction of Quark Pairs
We introduce a nonperturbative interaction for light-cone fluctuations
containing quarks and gluons. The interaction squeezes the transverse
size of these fluctuations in the photon and one does not need to simulate this
effect via effective quark masses. The strength of this interaction is fixed by
data. Data on diffractive dissociation of hadrons and photons show that the
nonperturbative interaction of gluons is much stronger. We fix the parameters
for the nonperturbative quark-gluon interaction by data for diffractive
dissociation to large masses (triple-Pomeron regime). This allows us to predict
nuclear shadowing for gluons which turns out to be not as strong as
perturbative QCD predicts. We expect a delayed onset of gluon shadowing at shadowing of quarks. Gluon shadowing turns out to be nearly scale
invariant up to virtualities due to presence of a semihard
scale characterizing the strong nonperturbative interaction of gluons. We use
the same concept to improve our description of gluon bremsstrahlung which is
related to the distribution function for a quark-gluon fluctuation and the
interaction cross section of a fluctuation with a nucleon. We expect
the nonperturbative interaction to suppress dramatically the gluon radiation at
small transverse momenta compared to perturbative calculations.Comment: 58 pages of Latex including 11 figures. Shadowing for soft gluons and
Fig. 6 are added as well as a few reference
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