Does Shadowing Relate to Nuclear Radius?

The A-dependence of nuclear shadowing in the partonic shadowing model is analysed. We point out that the nuclear shadowing is related to the average number of shadowed nucleons rather than the nuclear radius.     

Structure functions of bound nucleons: From the EMC effect to nuclear shadowing

We describe the nuclear structure functions in the whole range of the Bjorken variablex, by combining various effects in a many-step procedure. First, we present a QCD motivated model of nucleons, treated, in the limit of vanishingQ ², as bound states of three relativistic constituent quarks. Gluons and sea quarks are generated radiatively from the input valence quarks. All parton distributions are described in terms of the confinement (or nucleon's) radius. The results for free nucleons are in agreement with the experimental determinations. The structure functions of bound nucleons are calculated by assuming that the main effect of nucleon binding is stretching of nucleons. The larger size of bound nucleons lowers the valence momentum and enhances the radiatively generated glue and sea densities. In the small-x region the competitive mechanism of nuclear shadowing takes place. It also depends on the size of the nucleons. By combining stretching, shadowing and Fermi motion effects (the latter confined to very largex), the structure function ratio is well reproduced. Results are also presented for theA-dependence of the momentum integral of charged partons, the nuclear gluon distribution and the hadron-nuclei cross sections.     

Nuclear shadowing effect in relativistic heavy-ion collisions

Two complementary approaches are used in studying the nuclear shadowing effect in heavy-ion collisions at SIS/GSI beam energies within a hadronic transport model. By analysing simultaneously the average transverse momentum of nucleons and pions in the reaction plane as a function of rapidity, the shadowing effect is revealed as an anticorrelation of the average transverse momentum distributions of nucleons and pions in semicentral and peripheral collisions. While by studying the azimuthal angle distribution of pions with respect to the reaction plane, the shadowing effect appears as an azimuthal anisotropy with a preferential emission of pions perpendicular to the reaction plane. The dependence of the nuclear shadowing effect on the impact parameter and the beam energy is also studied.     

Note on shadowing and diffraction in deep-inelastic lepton scattering

We discuss the close relation between shadowing in deep-inelastic lepton-nucleus scattering and diffractive photo- and leptoproduction of hadrons from free nucleons. We show that the magnitude of nuclear shadowing at small Bjorken-x, as measured by the E665 and NMC collaborations, is directly related to HERA data on the amount of diffraction in the scattering from free nucleons. Received: 16 October 1998     

Phenomenology of nuclear shadowing in deep-inelastic scattering

We investigate shadowing effects in deep-inelastic scattering from nuclei at small valuesx < 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 smallx. 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 atx < 0.1. We compare our results with most of the recent data for a large variety of nuclei and examine in particular the Q² dependence of the shadowing effect. While the coherent interaction of low mass vector mesons causes a major part of the shadowing observed in the Q² range of current experiments, the coherent scattering of continuum quark-antiquark pairs is also important and guarantees a very weak overall Q² 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.     

Parton Evolution Model and the EMC Effect

The effect of the evolution of partons in nuclear environment to the structure function of nucleons bound in a nucleus is investigated.Together with the nuclear shadowing effect caused by the spatial overlap of partons from neighboring nucleons in the nucleus in the small x region,the ratio of the average nuclear structure function of nucleus ⁵⁶Fe to the deuteron structure function is calculated and a good agreement between the calculated results and the experimental data is obtained.     

Contact electromagnetic terms and nuclear shadowing of high-energy photons

The relevance of contact electromagnetic terms in the absorption of photons by nucleons and nuclei is pointed out. A model for computing those terms is presented. Results for the nuclear shadowing of photons are given.     

Shadowing in nuclear photoabsorption

Shadowing in nuclear photoabsorption is studied using a generalised vector dominance model, and the simplifying assumptions of previous calculations shown to be inadequate. Less shadowing is found than hitherto, with possible anti-shadowing indicated for x′ ? 0.07. The results are compatible with experiment within the substantial systematic errors.     

Target rescattering corrections to heavy flavour production off nuclei in the projectile fragmentation region

It is shown that at high energies secondary interactions of a produced particle with the nuclear target lead to shadowing, which is described by essentially the same formulas that follow from the low energy picture based on sequential interactions with target nucleons. No finite formation time effects can be seen.     

Nuclear dependence of structure functions in coordinate space

The momentum distributions of partons in bound nucleons are known to depend significantly on the size of the nucleus. The Fourier transform of the momentum (x _Bj) distribution measures the overlap between Fock components of the nucleon wave function which differ by a displacement of one parton along the light cone. The magnitude of the overlap thus determines the average range of mobility of the parton in the nucleon. By comparing the Fourier transforms of structure functions for several nuclei we study the dependence of quark mobility on nuclear size. We find a surprisingly small nuclear dependence (< 2% for He, C and Ca) for displacements t = z ? 2.5 fm, after which a nuclear suppression due to shadowing sets in. The nuclear effects observed in momentum space for x _Bj ? 0.4 can be understood as a reflection of only the large distance shadowing in coordinate space.     

Memory effects in the energy dissipation for slow collective nuclear motion

The energy dissipation in slow collective nuclear motion is considered as a combined effect of the diabatic production of particle-hole excitations and the subsequent equilibration by two-body collisions. Memory effects due to the long mean free path of the nucleons are treated analytically for an interacting Fermi gas within moving walls leading to a friction kernel (frequency-dependent friction coefficient) in the classical equation of collective motion.     

Meson fields, exchange currents and progenitor sum rules

The theory of the integrated photoabsorption cross section and the dipole sum rule is reviewed. A new progenitor sum rule, in which meson fields appear explicitly, is derived from a field theory of interacting pions and nucleons. The relation of this sum rule to previous results obtained in a potential theory is elucidated, and the role of neutron-proton correlations in nuclear ground states is discussed in terms of their effect on the dipole sum rule. The inclusion of ρ-mesons (again in an oversimplified theory) and their contribution to progenitor sum rules is also mentioned.     

INVESTIGATION OF THE NUCLEAR SHADOWING EFFECT IN THE SMALL x REGION

In this paper,we analyse the spatial overlapping behavior of sea quarks of neighboring nucleons in nuclei in the small x region and give a relation between the nuclear shadowing factor R(x,Q²,A) and the spatial overlapping factor ΔV_A(x)/V_A(x).By using this nuclear shadowing factor R(x,Q²,A),we calculate the ratios of the average nuclear structure functions of nuclei ¹²C、⁶³Cu and ¹¹⁶Sn,to the deuteron structure function and obtais a better agreement between the calculated result and the experimental data.     

Shadowing,dipions and the EMC effect

The EMC effect is thought to be a compound phenomenon that appears when nucleons are bound in a nucleus. The shadowing effect shows up in a very low-x range. It is possible that in low- and medium-x ranges there exist weak-binding regions caused by the nucleon distortion in the nucleus as well as by exchange pions among the nucleons and other pions in weak-binding regions. Both these types of pions will be in the dipion state because of the Coulomb field. The dipion effect contributes to the structure function of the nucleus. Nuclear binding energy and Fermi motion in medium- and higher-x ranges cause a shift of the ratio of the nucleus structure function to the deuteron one. Calculations show that better agreement with recent data is obtained if the above three effects are mixed.     

Quantum and thermal fluctuation effects on the photoabsorption spectra of clusters

A first principles approach for the calculation of photoabsorption cross sections of clusters and molecules is presented which includes both quantum and thermal fluctuations. The method relies on an ab initio path integral representation of the nuclear quantum motion in conjunction with time-dependent density-functional theory for electronic excitations. It is shown that quantum fluctuations of cold lithium clusters, such as Li8 below 50 K, affect significantly their photoabsorption spectra.     

Applications of progenitor sum rules in nuclear theory

The energy-weighted sum rules, obtained by taking matrix elements of double commutators with the nuclear Hamiltonian of appropriately defined densities, are didactically reviewed.Following a catalogue of applications such sum rules have found in theoretical nuclear physics, the progenitor sum rules (PSR's) are derived using standard commutator relations for second-quantized nucleon field operators. This leads to the extension of PSR's to coupled Fermi-Bose systems, and in particular, to a simple model of interacting mesons and nucleons. The effects of interactions are also discussed from the point of view of (static) exchange potentials. The relation of the dipole sum rule to nuclear photoabsorption is reviewed briefly.The application of PSR's to the calculation of longitudinal inelastic form factors, and the use of the “doorway hypothesis” is discussed in detail, together with the semiclassical collective-oscillation interpretation of these results proposed by Bertsch. Suzuki's very interesting applications of PSR to the evaluation of collective mass parameters, coupling constants, and polarization functions of the Bohr-Mottelson self-consistent mean-field theory are also given a detailed treatment. Certain miscellaneous applications of PSR to calculation of Coulombic isospin mixing, to energy shifts in isospin multiplets, and to induced moments and effective charges are collected and discussed in a unified manner. The review concludes with a discussion of the qualitative character of the nuclear photoabsorption strength below the meson production threshold, especially with regard to the question of to what extent the (γ, NN) reaction is sensitive to pair correlations in the nuclear ground state.     

Isotopic effect in the width of a giant dipole resonance in light nuclei

An isotopic effect in the widths of giant dipole resonances is established on the basis of an analysis of the latest systematics of photoabsorption cross sections for nuclei containing 12 to 65 nucleons. This effect arises owing to isospin splitting of a giant resonance and is enhanced by its configuration splitting.     

Fading out of J/ψ color transparency in high energy heavy ion peripheral collisions

We provide predictions for the J/ψ coherent production in the peripheral heavy ion collisions at LHC and RHIC using the leading twist model of nuclear shadowing based on the QCD factorization theorem for diffraction and the HERA hard diffraction data. We demonstrate that for LHC kinematics this model leads to a bump-shape distribution in rapidity which is suppressed overall as compared to the expectations of the color transparency regime by a factor 6. This is a significantly larger suppression than that expected within the impact parameter eikonal model. Thus we show that the interaction of spatially small wave package for which the total cross section of interaction with nucleons is small is still strongly shadowed by nuclear medium in high energy processes.