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DESY, Diss., 2003; (2003). doi:10.1016/j.physrep.2003.08.002, We give an overview of the theory for generalized parton distributions. Topics covered are their general properties and physical interpretation, the possibility to explore the three-dimensional structure of hadrons at parton level, their potential to unravel the spin structure of the nucleon, their role in small-x physics, and efforts to model their dynamics. We review our understanding of the reactions where generalized parton distributions occur, to leading power accuracy and beyond, and present strategies for phenomenological analysis. We emphasize the close connection between generalized parton distributions and generalized distribution amplitudes, whose properties and physics we also present. We finally discuss the use of these quantities for describing soft contributions to exclusive processes at large energy and momentum transfer., Published by Elsevier Science, North-Holland, Amsterdam [u.a.]

Universität Hamburg, Diss., 2003; 192 pp., (2003). doi:10.3204/DESY-THESIS-2003-037, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

6 pp. (2002)., I give a brief introduction to generalized Parton distributions and to the phenomenology of the exclusive processes where they can be measured.

Universität Hamburg, Diss., 2001; 174 pp., (2001). doi:10.3204/DESY-THESIS-2001-059, Published by Deutsches Elektronen-Synchrotron, DESY, Hamburg

Physical review / C 62(6), 064903 (2000). doi:10.1103/PhysRevC.62.064903, 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., Published by Inst., Woodbury, NY

6, Hamburg, Germany, 17 May 1999 - 20 May 1999; Physical review / D 60(7), 071501 (1999). doi:10.1103/PhysRevD.60.071501, In the following note, we will present an estimation of the single spin asymmetry in deeply virtual Compton scattering (DVCS) which directly allows one to test predictions of the ratio of the imaginary part of the amplitude in DIS to DVCS, as well as access the skewed parton distributions at small $x$ in the DGLAP region. We find it to be large for the HERA kinematics to be accessible in forthcoming runs with polarized electrons., Published by Inst., Woodbury, NY

Universität Hamburg, Diss., 1999; 91 pp., (1999). doi:10.3204/DESY-THESIS-1999-024, We analyse gluonic twist-four contributions to inclusive deep inelastic structure functions. For this, we estimate these contributions numerically in the first part. The results are based on the DLA contributions of gluon-operators which are expected to dominate at small XB. We, first, summarize relations of Ute two-e, three- and four-gluon amplitude and show how to extract the relevant twistfour. parts. Moreover, we are using experimental data on diffractive final states in order to fix free parameters within the initial distribution of the four-gluon amplitude. Therefore, it is also necessary to study relations between diffractive and inclusive amplitudes. This relation can be established with help of the AGK rules. Finally, we present our numerical results for three different choices of the input distributions. In the second part we consider gluonic higher-twist contributions from a more general point of view. Using a graphical method we can find expressions for gluonic twist-four contributions at leading order. For thls we consider diagrams with up to four t-channel gluons. The used scheme is an extension of the methods developed by R.K. Ellis, W. Furmanski, R. Petronzio (EFP). Making systematic use of the axial gauge, Ward identities and symmetry relations, we can write the results as explicitly gauge invariant expressions. Moreover, we can perform the factorization of matrix-elements of gluon-operators and coefficient functions.

Physical review / D 60(1), 014010 (1999). doi:10.1103/PhysRevD.60.014010, The amplitude for hard exclusive pseudoscalar meson electroproduction off nucleon (nuclear) targets is computed in QCD within the leading $\alpha_s \ln {Q^2/\lambda_{QCD}^{2}}$ approximation. We show that the distribution of recoil nucleons depends strongly on the angle between the momentum of the recoil nucleon and the polarization vector of the target (or outgoing nucleon). This dependence is especially sensitive to the spin flip skewed parton distribution (SPD) $\widetilde E$. We argue also that the scaling for this spin asymmetry sets in at lower Q^2 than that for the absolute cross section. Basing on the chiral quark-soliton model of the nucleon we estimate quantitatively the spin asymmetry. In the case of pi+ production this asymmetry is dominated at small t by the contribution of the pion pole in the isovector SPD $\widetilde E$ as required by PCAC. In the case of K0 production off a proton we find a large enhancement of the cross section as compared to the case of pi0 production. For the forward production of neutral pseudoscalar mesons off a deuteron target we find the cross section should be zero for the zero deuteron helicity (along the $\gamma^*D$ direction). We consider also cross sections of quasielastic processes off nuclei including the feasibility to implant K+,rho-mesons into nuclear volume., Published by Inst., Woodbury, NY

Universität Hamburg, Diss., 1999; 79 pp., (1999). doi:10.3204/DESY-THESIS-1999-004, A complete next-to-leading order calculation in QCD of one-particle-inclusive hadron production in deep-inelastic scattering is presented in this thesis. The calculations have been done for the exchange of a highly virtual photon and include corrections up to the order $O(\alpha^2_s)$. These reactions offer an excellent possibility to study perturbative impacts of the QCD like the hard-scattering cross-section, as well as perturbative aspects like fragmentation functions and parton distributions. The numerical calculation of the hard process, convoluted with these functions leads to one-particle inclusive cross-sections. These can be compared with experimantal results from the Hl- and ZEUS-collaborations.

149 pp. (1999).

196 pp. (1998)., The advent of the $ep$ collider HERA at the DESY research center has opened a wide kinematical wi11dow for the study of the Deep Inelastic Scattering (DIS) processes $e^+{p} \to e^+{X}$. Recent measurements of the structure function $F_2$ of the proton have confirmed the fact that the Bjorken scaling which arises from the naive Quark-Parton Model is broken in the limit where the scaling variable, $x$-Bjorken, is small. It also revealed that $F_2$ grows exponentially with $x$-Bjorken at low $x$. The standard picture of parton evolution, based on the DGLAP equations, reproduces this steep rise if it is used with an appropriate set of $F_2$ parametrizations. However these parametrizations iuclude 'ad-hoc' assumptions on the non-perturbative behavior or the structure functions and depend on a large set of phenomenological parameters. The steep rise can also be reproduced by using different perturbative treatment for the parton evolution. This treatment, based on the BFKL evolution equations, differs from the standard DGLAP picture by the lack of ordering in the transverse energy of the emitted partons. Unlike DGLAP, the BFKL picture reproduces the steep rise of $F_2$ without assuming any particular shape of the structure function in the non-perturbative domain. In order to gain an insight on parton dynamics in DIS at low $x$ and discriminate between these two pictures, jet observables are used as they are expected to be closely related to the hard scattering and depend only slightly on the hadronization effects. Two jet observables are studied here in more details: the azimuthal correlation between the two leading-order jets and the cross section of the forward jet production. The measured cross sections are corrected for detector effects and compared to several DIS Monte Carlo models and next-to-leading order simulations over a wide kinematic range. The results are evaluated in the lightof the BFKL and the DGLAP pictures. The experimental results are compared to other models of parton evolution as well, like the Colour Dipole Model (CDM), the Linked Dipole Chain (LDC) and the resolved photon model in DIS.

Universität Hamburg, Diss., 1998; 140 pp., (1998). doi:10.3204/PUBDB-2016-02735

University of Oxford, Diss., 1998; 132 pp., (1998). doi:10.3204/PUBDB-2016-02631

Physical review / D 57(5), 2806 - 2812 (1998). doi:10.1103/PhysRevD.57.2806, A next-to-leading order Monte Carlo program for the calculation of heavy quark cross sections in deeply inelastic scattering is described. Concentrating on charm quark and D^*(2010) production at HERA, several distributions are presented and their variation with respect to charm quark mass, parton distribution set, and renormalization-factorization scale is studied., Published by Inst., Woodbury, NY

440303 152 pp. (1997).

74 pp. (1996)., A description of the Fortran program HECTOR for a variety of semi-analytical calculations of radiative QED, QCD, and electroweak corrections to the double-differential cross sections of NC and CC deep inelastic charged lepton proton (or lepton deuteron) scattering is presented. HECTOR originates from the substantially improved and extended earlier programs HELIOS and TERAD91. It is mainly intended for applications at HERA or LEPxLHC, but may be used also for muon scattering in fixed target experiments. The QED corrections may be calculated in different sets of variables: leptonic, hadronic, mixed, Jaquet-Blondel, double angle etc. Besides the leading logarithmic approximation up to order O(alpha~2), exact order O(alpha) corrections and inclusive soft photon exponentiation are taken into account. The photoproduction region is also covered.

Universität Hamburg, Diss., 1996; 128 pp. (1996). = Universität Hamburg, Diss., 1996

23 pp. (1995)., Photoproduction of 2-jet events is studied with the H1 detector at HERA. Parton cross sections are extracted from the data by an unfolding method using leading order parton-jet correlations of a QCD generator. The gluon distribution in the photon is derived in the fractional momentum range 0.04 ⩽ x γ ⩽ 1 at the average factorization scale 75 GeV 2 .

8 pp. (1994)., Recently, the ZEUS Collaboration has reported on several remarkable properties of events with a large rapidity gap in deep inelastic scattering. We suggest that the mechanism underlying these events is the scattering of electrons off lumps of wee partons inside the proton. Based on an effective lagrangian approach the Q 2 -, x - and W -distributions are evaluated. For sufficiently small invariant mass of the detected hadronic system, the mechanism implies leading twist behaviour. The x - and W -distributions are determined by the Lipatov exponent which governs the behaviour of parton densities at small x .

The quark and gluon content of the Pomeron and the notion of a Pomeron structure function is discussed. The Pomeron is argued to be small object and, as a consequence, small-x gluon recombination effects could be sizeable. Ways to measure the Pomeron parton densities in pp and ep collisions are presented.