arXiv subjects

G. Piller

Publications and source records attributed to G. Piller.

30 records · Page 2Linked to original sources

arXiv1996

Diffractive rho^0 photo- and leptoproduction at high energies

We discuss the elastic photo- and leptoproduction of rho^0-mesons from nucleons at Q^2 < 1 GeV^2 as studied in recent experiments at HERA and FNAL. We find that the mass distribution of the measured π^+π^- pairs is determined to a large extent by the two-pion contribution to the photon spectral function as given by the pion form factor. With rising Q^2 the rate of diffractive events decreases and the π^+π^- mass distribution approaches a symmetric shape.

hep-ph↗

arXiv1996

Coherent $ρ$ Production from Polarized Deuterium

We discuss the coherent leptoproduction of vector mesons from polarized deuterium as a tool to investigate the evolution of small size quark-gluon configurations. Kinematic regions are determined where the final state interaction of the initially produced quark-gluon wave packet contributes dominantly to the production cross section. Two methods for an investigation of color coherence effects, which are appropriate for future experiments at HERMES, are suggested.

hep-ph↗

arXiv1996

Deep Inelastic Structure Functions in a Covariant Spectator Model

Deep-inelastic structure functions are studied within a covariant scalar diquark spectator model of the nucleon. Treating the target as a two-body bound state of a quark and a scalar diquark, the Bethe-Salpeter equation (BSE) for the bound state vertex function is solved in the ladder approximation. The valence quark distribution is discussed in terms of the solutions of the BSE.

hep-ph↗

arXiv1996

Polarized Deep-Inelastic Scattering from Nuclei -- A Relativistic Approach

A covariant formulation of spin-dependent deep-inelastic scattering from nuclei is presented. In the relativistic impulse approximation the inclusive nuclear cross section is described in terms of the amplitude for forward, virtual-photon scattering from an off-mass-shell nucleon. The general structure of the off-shell nucleon hadronic tensor is derived, and the leading behavior of the off-shell nucleon structure functions computed in the Bjorken limit. The formalism, which is valid for nucleons bound inside nuclei with spin 1/2 or 1, is applied to the case of the deuteron.

nucl-th↗

arXiv1996

Structure Functions of the Nucleon in a Covariant Scalar Spectator Model

Nucleon structure functions, as measured in deep-inelastic lepton scattering, are studied within a covariant scalar diquark spectator model. Regarding the nucleon as an approximate two-body bound state of a quark and diquark, the Bethe-Salpeter equation (BSE) for the bound state vertex function is solved in the ladder approximation. The valence quark distribution is discussed in terms of the solutions of the BSE.

hep-ph↗

arXiv1995

Inclusive Heavy-Flavor Production from Nuclei

We describe a light-cone wave function formulation for hadroproduction of heavy-flavors at high energies. At moderate values of $x_F$ heavy-flavor production can be viewed as a diffractive excitation of heavy quark-antiquark Fock states, present in the interacting gluon from the projectile. The approach developed here is well suited to address coherence effects in heavy-quark production from nuclei at small values of $x_{t} \lsim 0.1\cdot A^{-1/3}$.

hep-ph↗

arXiv1995

$J/ψ$ production as a probe of charge symmetry violations and nuclear corrections in parton distributions

We investigate the sensitivity of $J/Ψ$ production in proton-neutron and proton-deuteron collisions to charge symmetry violations in the parton distributions as well as to nuclear corrections. It is found that the effects of charge symmetry violations in the quark distributions are confined to large $x_F$ and difficult to measure in experiments currently being planned. However, from proton-deuteron experiments it should be possible to isolate nuclear corrections to the gluon distribution.

hep-ph↗

arXiv1995

Phenomenology of Nuclear Shadowing in Deep-Inelastic Scattering

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.

hep-ph↗

arXiv1995

SPIN-DEPENDENT NUCLEAR STRUCTURE FUNCTIONS: GENERAL APPROACH WITH APPLICATION TO THE DEUTERON

We study deep-inelastic scattering from polarized nuclei within a covariant framework. A clear connection is established between relativistic and non-relativistic limits, which enables a rigorous derivation of convolution formulae for the spin-dependent nuclear structure functions g_1^A and g_2^A in terms of off-mass-shell extrapolations of polarized nucleon structure functions, g_1^N and g_2^N. Approximate expressions for g_{1,2}^A are obtained by expanding the off-shell g_{1,2}^N 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.

hep-ph↗

arXiv1995

DEEP INELASTIC SCATTERING FROM POLARIZED DEUTERONS

The spin-dependent structure function of the deuteron, g_1D, is calculated within a covariant framework. The off-shell structure of the bound nucleon gives corrections to the convolution model at a level of half a percent for x below 0.7, increasing to more than five percent at larger x. Overall, the dominant source of error comes from the lack of knowledge associated with the deuteron D-state, which may introduce an uncertainty in the neutron spin structure function, g_1n, extracted from deuterium data of up to ten percent for x around 0.2.

hep-ph↗

arXiv1994

Quantum coherence in heavy-flavor production on nuclei

We describe a light-cone wave function formalism for hadroproduction of heavy-flavors via gluon-gluon fusion. The developed approach is well suited to address coherence effects in heavy-quark production on nuclei at small $x_{2} \lsim 0.1\cdot A^{-1/3}$. Nuclear attenuation in hadroproduction of heavy-flavors is found to be similar to shadowing effects for heavy-flavor structure functions in deep-inelastic scattering from nuclei. Nevertheless, remaining differences in the theoretical formulation of both processes imply corrections to factorization theorems.

hep-ph↗

arXiv1994

Shadowing, Binding and Off-Shell Effects in Nuclear Deep Inelastic Scattering

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 of $\bar qq$ 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.

nucl-th↗