Comparison of NNLO DIS scheme splitting functionswith results from exact gluon kinematics at small x

We consider the effect of exact gluon kinematics in the virtual photon-gluon impact factor at small x. By comparing with fixed order DIS scheme splitting and coefficient functions, we show that the exact kinematics results match the fixed order results well at each order, which suggests that they allow for an accurate NLL analysis of proton structure functions. We also present, available for the first time, x-space parameterisations of the NNLO DGLAP splitting functions in the DIS scheme, and also the longitudinal coefficients for neutral current scattering. Received: 21 July 2005, Revised: 14 September 2005, Published online: 25 November 2005     

Issues in the GPD Formulation of DVCS

The kinematics used in computing deeply virtual Compton scattering makes a dramatic difference in terms of the widely used reduced operators that define generalized parton distributions. We analyze this difference at tree-level.     

Kinematical Issues in the GPD Formulation of DVCS

The kinematics used in computing deeply virtual Compton scattering makes a significant difference in terms of the widely used reduced operators that define generalized parton distributions. We analyze this difference at tree-level.     

Measurement of dijet cross sections in <Emphasis Type="Italic">ep</Emphasis> interactions with a leading neutron at HERA

Measurements are reported of the production of dijet events with a leading neutron in ep interactions at HERA. Differential cross sections for photoproduction and deep inelastic scattering are presented as a function of several kinematic variables. Leading order QCD simulation programs are compared with the measurements. Models in which the real or virtual photon interacts with a parton of an exchanged pion are able to describe the data. Next-to-leading order perturbative QCD calculations based on pion exchange are found to be in good agreement with the measured cross sections. The fraction of leading neutron dijet events with respect to all dijet events is also determined. The dijet events with a leading neutron have a lower fraction of resolved photon processes than do the inclusive dijet data.Received: 31 January 2005, Revised: 7 March 2005, Published online: 28 April 2005     

Q2 evolution of the generalized Gerasimov-Drell-Hearn integral for the neutron using a 3He target

We present data on the inclusive scattering of polarized electrons from a polarized 3He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5 degrees. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference sigma(1/2)-sigma(3/2). We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q2 of 0.1-0.9 GeV2.     

The total virtual photoabsorption cross section,deeply virtual Compton scattering and vector-meson production

Based on the generic two-gluon-exchange dynamical mechanism for deeply inelastic scattering at low , we stress the intimate direct connection between the total virtual photoabsorption cross section, deeply virtual Compton scattering and vector-meson electroproduction. A simple expression for the cross section for deeply virtual Compton scattering is derived. Parameter-free predictions are obtained for deeply virtual Compton forward scattering and vector-meson forward production, once the parameters in the total virtual photoabsorption cross section are determined in a fit to the experimental data on deeply inelastic scattering. Our predictions are compared with the experimental data from HERA. Received: 16 February 2004, Revised: 16 July 2004, Published online: 2 September 2004 Supported by Deutsche Forschungsgemeinschaft, contract number schi 189/6-2. An erratum to this article is available at .     

Generalized parton distributions

We discuss how generalized parton distributions (GPDs) enter in a variety of hard exclusive processes such as deeply virtual Compton scattering (DVCS) and hard meson electroproduction reactions on the nucleon. We show some key observables which are sensitive to the various hadron structure aspects of the GPDs, and discuss their experimental status.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Fz Elastic and Compton scattering - 13.60.Le Meson production - 12.38.Bx Perturbative calculations     

Nuclear effects in leptoproduction of secondaries

We estimate the energy losses in the case of deep-inelastic scattering on nuclear targets in terms of the effective change of the virtual photon energy. Our phenomenological results are in reasonable agreement with theoretical calculations. The difference in secondary production processes in hard and soft interactions is discussed. The text was submitted by the authors in English.     

Can the total angular momentum of u-quarks in the nucleon be accessed at HERMES?

We investigate the possibility of acquiring information on the generalized parton distribution E and, through a model for E, also on the u-quark total angular momentum J_u by studying deeply virtual Compton scattering and hard exclusive ρ⁰ electroproduction on a transversely polarized hydrogen target at HERMES. It is found that a change in J_u from zero to 0.4 corresponds to a 4σ (2σ) difference in the calculated transverse target-spin asymmetry in deeply virtual Compton scattering (ρ⁰ electroproduction), where σ is the total experimental uncertainty. PACS 12.38.Bx, 13.60.Le     

Predictions for high-energy real and virtual photon–photon scattering from color dipole BFKL–Regge factorization

High-energy virtual photon–virtual photon scattering can be viewed as an interaction of small size color dipoles from the beam and target photons, which makes scattering at high energies (LEP, LEP200 and NLC) an indispensable probe of the short distance properties of the QCD pomeron exchange. Based on the color dipole representation, we investigate the consequences for the scattering of the incorporation of asymptotic freedom into the BFKL equation which makes the QCD pomeron a series of isolated poles in the angular momentum plane. The emerging color dipole BFKL–Regge factorization allows us to relate in a model-independent way the contributions of each BFKL pole to scattering and DIS off protons. Numerical predictions based on our early works on the color dipole BFKL phenomenology of DIS on protons are in good agreement with the experimental data on the photon structure function and the most recent data on the cross section from the OPAL and L3 experiments at LEP200. We discuss the role of non-perturbative dynamics and predict a pronounced effect of the Regge-factorization breaking due to large unfactorizable non-perturbative corrections to the perturbative vacuum exchange. We comment on the salient features of the BFKL–Regge expansion for scattering including the issue of the decoupling of subleading BFKL poles and the soft plus rightmost hard BFKL pole dominance. Received: 9 January 2001 / Revised version: 25 September 2001 / Published online: 7 December 2001     

New NLO parameterizations of the parton distributionsin real photons

We present new next-to-leading-order (NLO) sets of parton distributions in real photons based on a scheme-invariant definition of the non-perturbative input. We compare the theoretical predictions with LEP data and a best fit allows us to constrain the parameters of the distributions. The shape of the gluon distribution is poorly constrained and we consider the possibility to measure it in photoproduction experiments. Three parameterizations that aim to take into account the scattering of the LEP data are proposed and compared to other NLO parameterizations. Received: 4 February 2005, Revised: 10 June 2005, Published online: 20 September s2005     

Probing nucleon strange asymmetry from charm production in neutrino deep inelastic scattering

We propose a means to detect the nucleon strange quark-antiquark asymmetry, which is predicted as a non-perturbative effect, but still unchecked directly by available experiments. The difference for the and meson production cross sections in neutrino and antineutrino induced charged current deep inelastic scattering is illustrated to be sensitive to the nucleon strange asymmetry. A prospect is given and the effect due to the light quark fragmentation is also discussed for the extraction of the strange asymmetry in future experiments.Received: 23 April 2005, Published online: 27 July 2005PACS: 14.60.Pq, 12.15.Ff     

Twist-4 photon helicity-flip amplitude in DVCS on a nucleon in the Wandzura-Wilczek approximation

We computed the twist-4 part of the photon spin-flip amplitude in deeply virtual Compton scattering on a nucleon in the Wandzura–Wilczek approximation. We found a factorizable contribution, which arises from photon scattering on quarks with non-zero angular momentum along the collision axis. As the genuine twist-2 amplitude arises at the NLO, for moderate virtualities of the hard photon, GeV, a kinematical twist-4 correction can give a numerically important contribution to the photon helicity-flip amplitude. Received: 2 July 2001 / Revised version: 17 July 2001 / Published online: 31 August 2001     

Positrons — an alternative probe to electron scattering

Interaction of positrons with atoms and molecules differs from electron interaction due to the difference in polarity of the charge. This makes positrons an alternative tool to study atomic and molecular structure. Recent measurements of the total cross-sections for positron scattering at low energies on He, Ar, H₂, N₂, C₆H₆, C₆H₁₂, C₆H₇N carried out at Trento University [Karwasz et al., Acta Phys. Pol. 127, 666 (2005)] are discussed and compared to electron scattering results. All measured total cross-sections exhibit an increase with decreasing positron energy in the limit of zero energy; H₂, N₂, Ar, show regions of constant cross-section which are a few eV-wide, characteristic of scattering on a hard-sphere potential. Helium shows two resonant structures much below the positronium formation threshold. They may be attributed to virtual positronium formation. In conclusion, positron scattering is complementary to electron scattering. The total cross-sections do not show Ramsuaer minima but constant values, and new resonances appear.     

Double deeply virtual Compton scattering off the nucleon

We study the double deeply virtual Compton scattering (DDVCS) process off the nucleon, through the scattering of a space-like virtual photon with large virtuality resulting in the production of a time-like virtual photon, decaying into an e(+)e(-) pair. This process is expressed in the Bjorken regime in terms of generalized parton distributions (GPDs), and it is shown that by varying the invariant mass of the lepton pair, one can directly extract the GPDs from the observables. We give predictions for the DDVCS cross section and beam helicity asymmetry and discuss its experimental feasibility.     

The accelerated scaling attractor solution of the interacting agegraphic dark energy in Brans–Dicke theory

We study the deeply virtual Compton scattering off a spin-one particle, as the case for the coherent scattering off a deuteron target. We extend our approach, formulated initially for a spinless case, and discuss the role of twist three contributions for restoring the gauge invariance of the amplitude. Using twist three contributions and relations, which emanate from the QCD equations of motion, we derive the gauge invariant amplitude for the deeply virtual Compton scattering (DVCS) off hadrons with spin 1. Using the derived gauge invariant amplitude, the single spin asymmetry is discussed.     

Diffractive parton distributions from perturbative QCD

The asymptotic collinear factorisation theorem, which holds for diffractive deep-inelastic scattering, has important modifications in the sub-asymptotic HERA regime. We use perturbative QCD to quantify these modifications. The diffractive parton distributions are shown to satisfy an inhomogeneous evolution equation. We emphasise that it is necessary to include both the gluonic and sea-quark t-channel components of the perturbative pomeron. The corresponding pomeron-to-parton splitting functions are derived in the appendix.Received: 20 April 2005, Published online: 27 July 2005     

Low-energy theorem for virtual compton scattering and generalized sum rules of the nucleon

We formulate the low-energy theorem for virtual Compton scattering off a nucleon and examine its consequences for generalized nucleon polarizabilites. As a result of a new, model-independent definition of the low-energy limit for doubly virtual Compton scattering, all generalized sum rules of the nucleon have a continuous limit for real photons and obtain contributions from the t channel that were not included previously.     

Virtual Compton Scattering off Spin-Zero Particles at Low Energies

We discuss the low-energy behaviour of the virtual Compton scattering amplitude off a spin-zero target. We first compare various methods of obtaining a low-energy expression based either on the soft-photon approximation or the use of Ward-Takahashi identities. We point out that structure-dependent terms are defined with respect to a low-energy approximation of the pole terms, which commonly is separated from the full amplitude. We derive a general expression for the structure-dependent terms in an expansion in terms of the momenta and of the initial and final virtual photon, respectively, up to and including terms of order . At order two terms appear that are related to the usual electric and magnetic polarizabilities of real Compton scattering. At order we find nine new structures of which five can only be determined using virtual photons. Received October 23, 1997; accepted for publication December 29, 1997