This review outlines our present experimental knowledge and theoretical understanding of deepinelastic scattering on nuclear targets. The emphasis is primarily on nuclear coherence phenomena, such as shadowing, where the key physics issue is the exploration of hadronic and quark-gluon fluctuations of a high-energy virtual photon and their passage through the nuclear medium. New developments in polarized deep-inelastic scattering on nuclei are also discussed, and more conventional binding and Fermi motion effects are summarized. The report closes with a brief outlook on vector meson electroproduction, nuclear shadowing at very large Q 2 and the physics of high parton densities in QCD.

We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of mesons in terms of double/nonforward parton distribution functions. After reviewing some general features of nonforward nucleon matrix elements of twist-2 QCD string operators, we propose a phenomenological model for quark and gluon nonforward distribution functions. The corresponding QCD evolution equations are solved in the leading logarithmic approximation for flavor nonsinglet distributions. We derive explicit expressions for hard exclusive π 0 , η, and neutral vector meson production amplitudes and discuss general features of the corresponding cross sections. * Work supported in part by BMBF

We present a unified description of nuclear deep inelastic scattering (DIS) over the whole region 0 < x < 1 of the Bjorken variable. Our approach is based on a relativistically covariant formalism which uses analytical properties of quark correlators. In the laboratory frame it naturally incorporates two mechanisms of DIS: (I) scattering from quarks and antiquarks in the target and (II) production of quark-antiquark pairs followed by interactions with the target. We first calculate structure functions of the free nucleon and develop a model for the quark spectral functions. We show that mechanism (II) is responsible for the sea quark content of the nucleon while mechanism (I) governs the valence part of the nucleon structure functions. We find that the coherent interaction ofqq pairs with nucleons in the nucleus leads to shadowing at small x and discuss this effect in detail. In the large x region DIS takes place mainly on a single nucleon. There we focus on the derivation of the convolution model. We point out that the off-shell properties of the bound nucleon structure function give rise to sizable nuclear effects.

No abstract

We study deep-inelastic scattering from polarized nuclei within a covariant framework. A clear connection is established between relativistic and nonrelativistic limits, which enables a rigorous derivation of convolution formulae for the spin-dependent nuclear structure functions g A 1 and g A 2 in terms of offmass-shell extrapolations of polarized nucleon structure functions, g N 1 and g N 2 . Approximate expressions for g A 1,2 are obtained by expanding the offshell g N 1,2 about their on-shell limits. As an application of the formalism we consider nuclear effects in the deuteron, knowledge of which is necessary to obtain accurate information on the spin-dependent structure functions of the neutron.

We investigate shadowing effects in deep-inelastic scattering from nuclei at small values x < 0.1 of the Bjorken variable. Unifying aspects of generalized vector meson dominance and color transparency we first develop a model for deep-inelastic scattering from free nucleons at small x. In application to nuclear targets we find that the coherent interaction of quark-antiquark fluctuations with nucleons in a nucleus leads to the observed shadowing at x < 0.1. We compare our results with most of the recent data for a large variety of nuclei and examine in particular the Q 2 dependence of the shadowing effect. While the coherent interaction of low mass vector mesons causes a major part of the shadowing observed in the Q 2 range of current experiments, the coherent scattering of continuum quark-antiquark pairs is also important and guarantees a very weak overall Q 2 dependence of the effect. We also discuss shadowing in deuterium and its implications for the quark flavor structure of nucleons. Finally we comment on shadowing effects in high-energy photon-nucleus reactions with real photons.To be published in Z. Phys. A.

We have calculated the NLO corrections to the twist-2 part of the deeply-virtual Compton scattering amplitude. Our results for the transverse and antisymmetric parts agree with recent calculations by Ji and Osborne and by Belitsky and Müller. In addition we present NLO results for the longitudinal part of the amplitude. * Work supported in part by BMBF

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. It is emphasized that present infrared singular contributions cannot be regularized through phenomenological meson distribution amplitudes with suppressed end-point configurations. We point out that infrared divergences arise also from integrals over skewed nucleon parton distributions. In a phenomenological analysis of transverse vector meson production model-dependent regularizations have to be applied. If this procedure preserves the 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.PACS number͑s͒: 13.60. Le, 12.38.Bx Exclusive electroproduction of mesons from nucleons has become a topic of broad interest. Experimental and theoretical advances have supported this development. At high energies a large amount of data has become available from experiments at CERN ͓New Muon Collaboration ͑NMC͔͒ and DESY ͑HERA, HERMES͒ ͑for references see, e.g., ͓1,2͔͒. Further measurements were carried out at DESY, and discussed at CERN ͑COMPASS͒ ͓3,4͔ and TJNAF ͓5͔. From the theoretical side a factorization theorem proven in Ref. ͓6͔ set the basis for many investigations. It states that the underlying photon-parton subprocesses are dominated for longitudinally polarized photons and large photon virtualities, Q 2 ӷ⌳ QCD 2 , by short distances and, hence, can be calculated perturbatively.As already emphasized in ͓6͔, the interaction of transversely polarized photons cannot be treated in a framework based on QCD factorization. This is due to infrared sensitive contributions which, e.g., result from large size quarkantiquark configurations in the produced meson. A straightforward QCD analysis of transverse vector meson production, as done for longitudinal ones ͑see Refs. ͓7-13͔͒, is therefore not possible. On the other hand, expected new data will provide more detailed information on these processes and help to investigate strong interaction dynamics at large distances.Descriptions of exclusive meson production from transversely polarized photons rely on model assumptions which are needed to regularize the present, at least logarithmic, infrared singularities. The exploratory analysis presented in this note is based on a naive application of Wilson's operator product expansion. In this framework we find constraints on the vector meson production amplitude imposed by Lorentz covariance. Different vector meson distribution amplitudes, which enter the production amplitude, are related by Wandura-Wilczek-type relations ͓14,15͔. These have significant implications for the nature of the present infrared singularities.In particular we observe that infrared divergences arise from integrals over meson distribution amplitudes and from integrals over skewed nucleon parton distributions. Fu...