Power-like corrections and the determination of the gluon distribution

Power-suppressed corrections to parton evolution may affect the theoretical accuracy of current determinations of parton distributions. We study the role of multigluon-exchange terms in the extraction of the gluon distribution for the Large Hadron Collider (LHC). Working in the high-energy approximation, we analyze multi-gluon contributions in powers of 1/Q^2. We find a moderate, negative correction to the structure function's derivative d F_2 / d \ln Q^2, characterized by a slow fall-off in the region of low to medium Q^2 relevant for determinations of the gluon at small momentum fractions

Comments on Exclusive Electroproduction of Transversely Polarized Vector Mesons

We discuss the electroproduction of light vector mesons from transversely polarized photons. Here QCD factorization cannot be applied as shown explicitly in a leading order calculation of corresponding Feynman diagrams. It is emphasized that present infrared singular contributions cannot be regularized through phenomenological meson distribution amplitudes with suppressed endpoint configurations. We point out that infrared divergencies arise also from integrals over skewed parton distributions of the nucleons. In a phenomenological analysis of transverse vector meson production model dependent regularizations have to be applied. If this procedure preserves the analytic structure suggested by a leading order calculation of Feynman diagrams, one obtains contributions from nucleon parton distributions and their derivatives. In particular polarized gluons enter only through their derivative

Study of Nondiagonal Parton Distribution Models

In this paper we examine predictions from different models of nondiagonal parton distributions. This will be achieved by examining whether certain predictions of relationships between diagonal and nondiagonal parton distributions also hold after having evolved the different distributions.Comment: 17 pages, 6 figures, RevTex. Some figures have been corrected due to a recently discovered small error in the evolution cod

Transversity GPD in photo- and electroproduction of two vector mesons

The chiral-odd generalized parton distribution (GPD), or transversity GPD, of the nucleon can be accessed experimentally through the photo- or electroproduction of two vector mesons on a polarized nucleon target, \gamma^(*) N --> \rho_1 \rho_2 N', where \rho_1 is produced at large transverse momentum, \rho_2 is transversely polarized, and the mesons are separated by a large rapidity gap. We predict the cross section for this process for both transverse and longitudinal \rho_{2} production. To this end we propose a model for the transversity GPD H_T(x,\xi,t), and give an estimate of the relative sizes of the transverse and longitudinal \rho_{2} cross sections. We show that a dedicated experiment at high energy should be able to measure the transversity content of the proton.Comment: 15 pages, 9 figures. Extended discussion of experimental prospects in the conclusions. Version to appear in EPJ

On the analytical approximation to the GLAP evolution at small x and moderate Q^2

Comparing the numerically evaluated solution to the leading order GLAP equations with its analytical small-x approximation we have found that in the domain covered by a large fraction of the HERA data the analytic approximation has to be augmented by the formally non-leading term which has been usually neglected. The corrected formula fits the data much better and provides a natural explanation of some of the deviations from the $\sigma$ scaling observed in the HERA kinematical range.Comment: LaTeX, 13 pages including 6 figures, figure 5 replaced, to appear in Phys. Lett.

The Status of Lattice Calculations of the Nucleon Structure Functions

We review our progress on the lattice calculation of low moments of both the unpolarised and polarised nucleon structure functions.Comment: 6 pages, contribution to 29th International Symposium on the Theory of Elementary Particles, Buckow, Germany, (29 August - 2 September 1995). 6 pages, Latex, requires espcrc2.sty, epsf.st

Neutrino induced hard exclusive D_s production

Motivated by the possibility to use high intensity neutrino beams for neutrino-nucleon scattering experiments we analyze charged current induced exclusive meson production within the framework of generalized parton distributions. The cross section for hard exclusive D_s production is estimated in this formalism to leading order in QCD. The integrated cross section proves to be sizable. It is shown that the considered process is well suited to provide novel information on the gluon structure of nucleons, which is contained in the generalized gluon parton distribution.Comment: 7 pages; modified plots in Fig. 3; accepted for publication in Phys. Lett.

Generalized Parton Distributions -- theoretical review --

In this talk an introduction to generalized parton distributions is given. Recent developments are shortly reviewed, including non-perturbative calculations, phenomenological aspects and evaluation of higher order perturbative and power corrections.Comment: 11 pages, 5 figure

Combining Lattice QCD Results with Regge Phenomenology in a Description of Quark Distribution Functions

The most striking feature of quark distribution functions transformed to the longitudinal distance representation is the recognizable separation of small and large longitudinal distances. While the former are responsible for the average properties of parton distributions, the latter can be shown to determine specifically their small-$x$ behavior. In this paper we demonstrate how the distribution at intermediate longitudinal distances can be approximated by taking into account constraints which follow from the general properties of parton densities, such as their support and behavior at $x \to 1$. We show that the combined description of small, intermediate, and large longitudinal distances allows a good approximation of both shape and magnitude of parton distribution functions. As an application we have calculated low-virtuality C even and odd (valence) u and d quark parton densities of the nucleon and the C-even transversity distribution $h_1(x)$, combining recent QCD sum rules and lattice QCD results with phenomenological information about their small-$x$ behavior.Comment: LaTeX, 17 pages including 7 figures, shorter version will appear in Phys. Lett.