23 research outputs found
A calculation of the Lepage-Mackenzie scale for the lattice axial and vector currents
We calculate the perturbative scales (q*) for the axial and vector currents
for the Wilson action, with and without tadpole improvement, using Lepage and
Mackenzie's formalism. The scale for the pseudoscalar density (times the mass)
is computed as well. Contrary to naive expectation, tadpole improvement reduces
q* by only a small amount for the operators we consider. We also discuss the
use of a nonperturbative coupling to calculate the perturbative scale.Comment: 13 pages. One postscript figur
One-Loop Factorization of the Nucleon g_2-Structure Function in the Non-Singlet Case
We consider the one-loop factorization of the simplest twist-three process:
inclusive deep-inelastic scattering of longitudinally-polarized leptons on a
transversely-polarized nucleon target. By studying the Compton amplitudes for
certain quark and gluon states at one loop, we find the coefficient functions
for the non-singlet twist-three distributions in the factorization formula of
g_2(x_B,Q^2). The result marks the first step towards a next-to-leading order
(NLO) formalism for this transverse-spin-dependent structure function of the
nucleon.Comment: 14 pages, revtex4, four figures included, minor change
A Poincare-Covariant Parton Cascade Model for Ultrarelativistic Heavy-Ion Reactions
We present a new cascade-type microscopic simulation of nucleus-nucleus
collisions at RHIC energies. The basic elements are partons (quarks and gluons)
moving in 8N-dimensional phase space according to Poincare-covariant dynamics.
The parton-parton scattering cross sections used in the model are computed
within perturbative QCD in the tree-level approximation. The Q^2 dependence of
the structure functions is included by an implementation of the DGLAP mechanism
suitable for a cascade, so that the number of partons is not static, but varies
in space and time as the collision of two nuclei evolves. The resulting parton
distributions are presented, and meaningful comparisons with experimental data
are discussed.Comment: 30 pages. 11 figures. Submitted to Phys.Rev.
Off-Forward Parton Distributions
Recently, there have been some interesting developments involving off-forward
parton distributions of the nucleon, deeply virtual Compton scattering, and
hard diffractive vector-meson production. These developments are triggered by
the realization that the off-forward distributions contain information about
the internal spin structure of the nucleon and that diffractive
electroproduction of vector mesons depends on these unconventional
distributions. This paper gives a brief overview of the recent developments
kt - factorization and CCFM - the solution for describing the hadronic final states - everywhere ?
The basic ideas of kt-factorization and CCFM parton evolution is discussed.
The unintegrated gluon densities, obtained from CCFM fits to the proton
structure function data at HERA are used to predict hadronic final state cross
sections like jet production at HERA, but also comparisons with recent
measurements of heavy quark production at the Tevatron are presented. Finally,
the kt-factorization approach is applied to Higgs production at high energy
hadron hadron colliders and the transverse momentum spectrum of Higgs
production at the LHC is calculated.Comment: to be published in MPLA, replaced with new reference
Charges in Gauge Theories
In this article we investigate charged particles in gauge theories. After
reviewing the physical and theoretical problems, a method to construct charged
particles is presented. Explicit solutions are found in the Abelian theory and
a physical interpretation is given. These solutions and our interpretation of
these variables as the true degrees of freedom for charged particles, are then
tested in the perturbative domain and are demonstrated to yield infra-red
finite, on-shell Green's functions at all orders of perturbation theory. The
extension to collinear divergences is studied and it is shown that this method
applies to the case of massless charged particles. The application of these
constructions to the charged sectors of the standard model is reviewed and we
conclude with a discussion of the successes achieved so far in this programme
and a list of open questions.Comment: 47 pages, LaTeX, 14 figures, uses feynmp, necessary Metapost files
included. Review to appear in Pramana, Journal of Physics. Minor LaTeX change
to make page numbers visible on "Letter" paper forma
A multiloop improvement of non-singlet QCD evolution equations
An approach is elaborated for calculation of "all loop" contributions to the
non-singlet evolution kernels from the diagrams with renormalon chain
insertions. Closed expressions are obtained for sums of contributions to
kernels for the DGLAP equation and for the "nonforward" ER-BL
equation from these diagrams that dominate for a large value of , the
first -function coefficient. Calculations are performed in the covariant
-gauge in a MS-like scheme. It is established that a special choice of the
gauge parameter generalizes the standard "naive nonabelianization"
approximation. The solutions are obtained to the ER-BL evolution equation
(taken at the "all loop" improved kernel), which are in form similar to
one-loop solutions. A consequence for QCD descriptions of hard processes and
the benefits and incompleteness of the approach are briefly discussed.Comment: 13 pages, revtex, 2 figures are enclosed as eps-file, the text style
and figures are corrected following version, accepted for publication to
Phys. Rev.
Polarized Virtual Photon Structure Function and Twist-3 Effects in QCD
We investigate the twist-3 effects in the polarized virtual photon structure.
The structure functions and of polarized photon could
be experimentally studied in the future polarized or colliders.
The leading contributions to are the twist-2 effects, while
another structure function , which only exists for the virtual
photon target, receives not only the twist-2 but also twist-3 contributions. We
first show that the twist-3 effects actually exist in the box-diagram
contributions and we extract the twist-3 part, which can also be reproduced by
the pure QED operator product expansion. We then calculate the non-trivial
lowest moment () of the twist-3 contribution to in QCD. For
large (the number of colors), the QCD analysis of the twist-3 effects in
the flavor nonsinglet part of becomes tractable and we can obtain
its moments in a compact form for all .Comment: 27 pages, LaTeX, 9 eps figures, eqsection.sty file included, Appendix
A added, some minor changes for Fig.
Solution of the Kwiecinski evolution equations for unintegrated parton distributions using the Mellin transform
The Kwiecinski equations for the QCD evolution of the unintegrated parton
distributions in the transverse-coordinate space (b) are analyzed with the help
of the Mellin-transform method. The equations are solved numerically in the
general case, as well as in a small-b expansion which converges fast for b
Lambda_QCD sufficiently small. We also discuss the asymptotic limit of large bQ
and show that the distributions generated by the evolution decrease with b
according to a power law. Numerical results are presented for the pion
distributions with a simple valence-like initial condition at the low scale,
following from chiral large-N_c quark models. We use two models: the Spectral
Quark Model and the Nambu--Jona-Lasinio model. Formal aspects of the equations,
such as the analytic form of the b-dependent anomalous dimensions, their
analytic structure, as well as the limits of unintegrated parton densities at x
-> 0, x -> 1, and at large b, are discussed in detail. The effect of spreading
of the transverse momentum with the increasing scale is confirmed, with
growing asymptotically as Q^2 alpha(Q^2). Approximate formulas for
for each parton species is given, which may be used in practical
applications.Comment: 18 pages, 6 figures, RevTe
Heavy Quark Photoproduction in k_T Factorization Approach
We investigate the heavy quark photoproduction based on the k_T factorization
approach, focusing on the results from the saturation model. The deviations in
the results using the unintegrated gluon distribution considering the
saturation model and the derivative of the collinear gluon distribution are
analysed. Total cross sections and p_T distributions are analysed in detail,
setting the deviations between the color dipole approximation and the complete
semihard approach.Comment: 18 pages, 7 figures, minor changes, references added. Accepted for
publication in Phys. Rev.