13 research outputs found
A QCD analysis of HERA and fixed target structure function data
The parton momentum densities in the proton are obtained from a NLO QCD
analysis of structure functions measured by HERA and fixed target experiments.
The uncertainties in these parton densities, the structure functions and
related cross sections are estimated from the experimental errors, taking into
account all correlations. Standard Model predictions for the charged current
Born cross sections at large x and Q2 are calculated and compared with recent
data from ZEUS.Comment: 4 pages LaTeX, 5 eps figures, uses npb.sty. Presented at DIS99,
Zeuthen, Germany, April 19-23, 199
Singular terms of helicity amplitudes at one-loop in QCD and the soft limit of the cross sections of multi-parton processes
We describe a general method that enables us to obtain all the singular terms
of helicity amplitudes of n-parton processes at one loop. The algorithm uses
helicity amplitudes at tree level and simple color algebra.
We illustrate the method by calculating the singular part of the one loop
helicity amplitudes of all parton subprocesses. The results are used
to derive the soft gluon limit of the cross sections of all parton
scattering subprocesses which provide a useful initial condition for the
angular ordering approximation to coherent multiple soft gluon emission,
incorporated in existing Monte Carlo simulation programs.Comment: Latex,13 pages, ETH-TH/94-
NLO corrections to the BFKL equation in QCD and in supersymmetric gauge theories
We study next-to-leading corrections to the integral kernel of the BFKL
equation for high energy cross-sections in QCD and in supersymmetric gauge
theories. The eigenvalue of the BFKL kernel is calculated in an analytic form
as a function of the anomalous dimension \gamma of the local gauge-invariant
operators and their conformal spin n. For the case of an extended N=4 SUSY the
kernel is significantly simplified. In particular, the terms non-analytic in n
are canceled. We discuss the relation between the DGLAP and BFKL equations in
the N=4 model.Comment: Latex, 26 pages, typos corrected, to be published in Nucl.Phys.
Isolated Prompt Photon Production in Hadronic Final States of Annihilation
We provide complete analytic expressions for the isolated prompt photon
production cross section in annihilation reactions through one-loop
order in quantum chromodynamics (QCD) perturbation theory. Functional
dependences on the isolation cone size and isolation energy parameter
are derived. The energy dependence as well as the full angular
dependence of the cross section on are displayed, where
specifies the direction of the photon with respect to the
collision axis. We point out that conventional perturbative QCD
factorization breaks down for isolated photon production in
annihilation reactions in a specific region of phase space. We discuss the
implications of this breakdown for the extraction of fragmentation functions
from annihilation data and for computations of prompt photon
production in hadron-hadron reactions.Comment: 54 pages RevTeX plus 19 postscript figures submitted together in one
compressed fil
Three-loop universal anomalous dimension of the Wilson operators in N=4 SUSY Yang-Mills model
We present results for the three-loop universal anomalous dimension of Wilson
twist-2 operators in the N=4 Supersymmetric Yang-Mills model. These results are
obtained by extracting the most complicated contributions from the three loop
non-singlet anomalous dimensions in QCD which were calculated recently. Their
singularities at j=1 agree with the predictions obtained from the BFKL equation
for N=4 SYM in the next-to-leading order. The asymptotics of universal
anomalous dimension at large j is in an agreement with the expectations based
on an interpolation between weak and strong coupling regimes in the framework
of the AdS/CFT correspondence.Comment: LaTeX file, 13 pages, no figures. Some corrections, additional
remarks and references. In the last version the analysis of anomalous
dimension at j -> 2 was improve
Inclusive Prompt Photon Production in Hadronic Final States of Annihilation
We provide complete analytic expressions for the inclusive prompt photon
production cross section in hadronic final states of annihilation
reactions through one-loop order in quantum chromodynamics perturbation theory.
Computed explicitly are direct photon production through first order in the
electromagnetic strength and the quark-to-photon and
gluon-to-photon fragmentation contributions through first order in the strong
coupling . The full angular dependence of the cross sections is
displayed, separated into transverse and
longitudinal components, where
specifies the direction of the photon with respect to the collision
axis. We discuss extraction of fragmentation functions from data.Comment: 40 pages, RevTex, 30 figures in postscript available in a separate
fil
DGLAP and BFKL evolution equations in the N=4 supersymmetric gauge theory
We derive the DGLAP and BFKL evolution equations in the N=4 supersymmetric
gauge theory in the next-to-leading approximation. The eigenvalue of the BFKL
kernel in this model turns out to be an analytic function of the conformal spin
|n|. Its analytic continuation to negative |n| in the leading logarithmic
approximation allows us to obtain residues of anomalous dimensions \gamma of
twist-2 operators in the non-physical points j=0,-1,... from the BFKL equation
in an agreement with their direct calculation from the DGLAP equation.
Moreover, in the multi-color limit of the N=4 model the BFKL and DGLAP dynamics
in the leading logarithmic approximation is integrable for an arbitrary number
of particles. In the next-to-leading approximation the holomorphic separability
of the Pomeron hamiltonian is violated, but the corresponding Bethe-Salpeter
kernel has the property of a hermitian separability. The main singularities of
anomalous dimensions \gamma at j=-r obtained from the BFKL and DGLAP equations
in the next-to-leading approximation coincide but our accuracy is not enough to
verify an agreement for residues of subleading poles.Comment: 45 pages, latex. In the last version the expression (16) for the
t-channel partial wave of the process e+e- --> \mu+\mu- in the
double-logarithmic approximation at QED is corrected and its derivation is
given in the Appendix
Statistical approach for unpolarized fragmentation functions for the octet baryons
A statistical model for the parton distributions in the nucleon has proven
its efficiency in the analysis of deep inelastic scattering data, so we propose
to extend this approach to the description of unpolarized fragmentation
functions for the octet baryons. The characteristics of the model are
determined by using some data on the inclusive production of proton and
in unpolarized deep inelastic scattering and a next-to-leading
analysis of the available experimental data on the production of unpolarized
octet baryons in annihilation. Our results show that both parton
distributions and fragmentation functions are compatible with the statistical
approach, in terms of a few free parameters, whose interpretation will be
discussed.Comment: 14 pages, 7 eps figures, to appear in Phys. Rev.
Small-x physics in perturbative QCD
We review the parton model and the Regge approach to the QCD description of
the deep-inelastic scattering at the small Bjorken variable and
demonstrate their relation with the DGLAP and BFKL evolution equations. It is
shown, that in the leading logarithmic approximation the gluon is reggeized and
the pomeron is a compound state of two reggeized gluons. The conformal
invariance of the BFKL pomeron in the impact parameter space is used to
investigate the scattering amplitudes at high energies and fixed momentum
transfers. The remarkable properties of the Schr\"odinger equation for compound
states of an arbitrary number of reggeized gluons in the multi-colour QCD are
reviewed. The gauge-invariant effective action describing the gluon-Reggeon
interactions is constructed. The known next-to-leading corrections to the QCD
pomeron are discussed.Comment: review to be published in Phys. Re