arXiv subjects

B. Z. Kopeliovich

Publications and source records attributed to B. Z. Kopeliovich.

172 records · Page 10Linked to original sources

arXiv1996

Interplay of Soft and Hard Interactions in Nuclear Shadowing at High $Q^2$ and Low x

Nuclear shadowing corrections are dominated by soft interaction and grow as function of $1/x$ more slowly than the single scattering term, which has an essential contribution from hard interaction. Therefore, we predict vanishing nuclear shadowing at very low $x$ provided that $Q^2$ is high and fixed. At the same time, at medium and low $Q^2$, nuclear shadowing grows with $1/x$ as is well known for soft hadronic interactions.

nucl-th↗

arXiv1996

Inelastic $J/Ψ$ Photoproduction off Nuclei: Gluon Enhancement or Double Color Exchange?

The nuclear enhancement observed in inelastic photoproduction of $J/Ψ$ should not be interpreted as evidence for an increased gluon density in nuclei. The nuclear suppression of the production rate due to initial and final state interactions is calculated and a novel two-step color exchange process is proposed, which is able to explain the data.

nucl-th↗

arXiv1996

Multiple Scattering Theory for the Photoproduction of Vector Mesons off Nuclei

The integrated cross section for the incoherent photoproduction of vector mesons on nuclei $γ^* A \to VX$, $X\not=A$, is calculated within Glauber theory and as a function of the photon energy. The inverse of the longitudinal momentum transfer is called coherence length $l_c$ and depends on the virtuality and the energy of the photon. Nuclear transmission factors strongly depend on $l_c/R_A$ ($R_A$ is the nuclear radius) and this effect may interfere with the search for color transparency effects.

nucl-th↗

arXiv1996

Novel Mechanism of Nucleon Stopping in Heavy Ion Collisions

When a diquark does not fragment directly but breaks in such a way that only one of its quarks gets into the produced baryon, the latter is produced closer to mid rapidities. The relative size of this diquark breaking component increases quite fast with increasing energy. We show that at a given energy it also increases with the atomic mass number and with the centrality of the collision and that it allows to explain the rapidity distribution of the net baryon number ($p$-$\bar{p}$) in $SS$ central collisions. Predictions for $Pb$-$Pb$ collisions are presented.

hep-ph↗

arXiv1996

Deep-Inelastic Electroproduction of Neutrons in the Proton Fragmentation Region

Experiments at HERA looking for deep-inelastic electroproduction of neutrons in the proton fragmentation region are in progress. They are aimed to measure the pion structure function at small Bjorken $x$. The important condition for such a study is to establish under what kinematical conditions the dominance of the pion-pole graph in the process is guaranteed. We analyse other sources of theleading neutron, in order to figure out the kinematical region where the one-pion exchange dominates.

hep-ph↗

arXiv1995

Where is the Baseline for Color Transparency Studies with Moderate Energy Electron Beams?

Study of color transparency (CT) effects at moderate energies is more problematic than is usually supposed. Onset of CT can be imitated by other mechanisms, which contain no explicit QCD dynamics. In the case of the $(e,e'p)$ reaction the standard inelastic shadowing well known in the pre-QCD era, causes a substantial growth of nuclear transparency with $Q^2$ and a deviation from the Glauber model, analogous to what is assumed to be a signal of CT. In the case of exclusive virtual photoproduction of vector mesons, CT is expected to manifest itself as an increase of nuclear transparency with $Q^2$ in production of the ground states and as an abnormal nuclear enhancement for the radial excitations. We demonstrate that analogous $Q^2$-dependence can be caused at moderate energies by the variation of so called coherence length, which is an interference effect, even in the framework of the vector dominance model. One should disentangle the real and the mock CT effects in experiments planned at CEBAF, at the HERMES spectrometer, or at the future electron facility ELFE.

nucl-th↗

arXiv1995

Color Transparency or the Standard Inelastic Shadowing?

The standard first-order inelastic correction (IC) well known in the pre-QCD era, causes a rising $Q^2$-dependence of nuclear transparency in the quasielastic electron scattering, $A(e,e'p)A'$, at moderate $Q^2$, similar to what is supposed to be the onset of color transparency (CT). Although IC is a part of the whole pattern of CT, it contains no explicit QCD dynamics. Evaluation of this correction is based on experimental data on diffraction dissociation and is independent of whether CP phenomenon exists or not. The growth of nuclear transparency is numerically comparable with the expected signal of CT up to about $Q^2\approx 20\ GeV^2$. Analogous effect in $A(p,2p)A'$ reaction is estimated as well.

hep-ph↗

arXiv1993

Decisive test of color transparency in exclusive electroproduction of vector mesons

The exclusive production of vector mesons in deep inelastic scattering is a hard scattering process with the well controlled size of quark configurations which dominate the production amplitude. This allows an unambiguous prediction of color transparency effects in the coherent and incoherent production of vector mesons on nuclei. We demonstrate how the very mechanism of color transparency leads to a belated onset of color transparency effects as a function of $Q^{2}$. We conclude that the $Q^{2}$ dependence of the exclusive $ρ^{0}$-meson production on nuclei and nucleons observed in the Fermilab E665 experiment gives a solid evidence for the onset of color transparency. We propose the scaling relation between the $ρ^{0}$ and the $J/Ψ$ production, which further tests the mechanism of color transparency in exclusive (virtual) photoproduction.

hep-ph↗

arXiv1993

Colour Screening, Quark Propagation in Nuclear Matter and the Broadening of the Momentum Distribution of Drell-Yan Pairs

We calculate the broadening of the transverse momentum distribution of a quark propagating through nuclear matter. Colour screening plays a fundamental role in that it cuts off quark-nucleon interactions with soft gluons. The mean transverse momentum of the quark acquired along its trajectory, observed via Drell-Yan pairs, is related to it the ratio of the total inelastic meson-nucleon cross section it to the meson mean squared radius. Parameter-free calculations agree with the data.

hep-ph↗

arXiv1993

Novel Color Transparency Effect: Scanning the Wave Function of Vector Mesons

We demonstrate how the virtual photoproduction of vector mesons on nuclei scans the wave function of vector mesons from the large non-perturbative transverse size $ρ\sim R_{V}$ down to the small perturbative size $ρ\sim 1/\sqrt{Q^{2}}$. Thee mechanism of scanning is based on color transparency and QCD predicted spatial wave function of quark-antiquark fluctuations of virtual photons. A rich, energy- and $Q^{2}$-dependent, pattern of the nuclear shadowing and antishadowing is predicted, which can be tested at the European Electron Facility and SLAC.

hep-ph↗