Nucleon Form Factors '99
We review recent progress in the experimental knowledge of and theoretical speculations about nucleon form factors, with special emphasis on the large Q2 region.
hep-ph↗
arXiv subjects
Bernard Pire
Publications and source records attributed to Bernard Pire.
Exclusive electroproduction of vector mesons and transversity distributions
We show that the leading twist contribution to exclusive electroproduction of transversely polarized vector mesons vanishes at all orders in perturbation theory. Therefore one cannot extract information on the quark transversity distribution of the nucleon from this reaction. In turn one has that the produced vector meson is purely longitudinal in the large-Q^2 limit, which provides a new test of the dominance of leading twist at finite Q^2.
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Physics observables for color transparency
The physics observables dedicated to the study of color transparency are diverse. After a brief pedagogical introduction, we emphasize the complementarity of the nuclear filtering and color transparency concepts. The importance of quantum interferences leads to suspect pictures based on the preparation of a squeezed state of small transverse size leading to a rapid transverse expansion. The different roles of heavy and light nuclei are emphasized. The possibility of color transparency tests in the vacuum is stressed. Such a test may be quickly addressed in the virtual Compton scattering reactions at CEBAF or HERMES energies.
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Color Transparency at ELFE
We review the presentations given in the Color Transparency parallel session held during the second ELFE workshop.
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Exclusive reactions in QCD
We review the theory of hard exclusive scattering in Quantum Chromodynamics. After recalling the classical counting rules which describe the leading scale dependence of form factors and exclusive cross-sections at fixed angle, the pedagogical example of the pion form factor is developped in some detail in order to show explicitely what factorization means in the QCD framework. The picture generalizes to many hard reactions which are at the heart of the ELFE project. We briefly present the concepts of color transparency.
nucl-th↗
Color Transparency Physics: A New Domain of Strong Interactions
The concept of color transparency is introduced. This new feature of QCD is characteristic of a gauge theory. It enables strong interactions to be studied in a new domain: scattering amplitudes of transversally small color singlet objects. Experimental data are still few and somehow inconclusive. Future prospects are briefly discussed. This is a frontier problem of QCD which deserves much attention from both theorists and experimentalists in the coming years.
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The Physics of ELFE
This paper presents an overview of the physics program of the 15-30~GeV continuous beam electron facility proposed by the European community of nuclear physicists to study the quark and gluon structure of hadrons. The goal of this new facility is to explore the quark structure of matter by exclusive and semi-inclusive electron scattering from nuclear targets.
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Physics with ELFE
A 15-30 GeV continuous beam electron facility has been proposed by nuclear physicists in Europe to study how color forces build up hadrons from quarks and gluons. This project and its physics case are briefly reviewed. The recommendations of NuPECC, the Nuclear Physics Committee of the European Science Foundation are presented.
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Quantum Color Transparency and Nuclear Filtering
Color transparency is the proposal that under certain circumstances the strong interactions can be reduced in magnitude. We give a comprehensive review of the physics, which hinges on the interface of perturbative QCD with non--perturbative strong interactions. Color transparency is complementary to {\it nuclear filtering}, which is the conversion of quark wave functions in hadrons to small transverse space dimensions by interaction with a nuclear medium. We review current approaches, including pictures based on modeling the time evolution of hadronic wave--packets as well as the use of light cone matrix elements. Spin plays an intrinsic role in testing and understanding the physics and is discussed at length. We emphasize the use of data analysis procedures which have minimal model dependence. We also review existing experimental data and the experimental program planned at various facilities. The subject has strong scientific complementarity and potential to make progress in exploring hadron physics at current and future facilities.
hep-ph↗
Hard Exclusive Scattering in QCD
We review the theory of hard exclusive scattering in Quantum Chromodynamics. After recalling the classical counting rules which describe the leading scale dependence of form factors and elastic reactions at fixed angle, the pedagogical example of the pion form factor is developped in some detail in order to show explicitely what factorization means in the QCD framework. The inclusion of transverse degrees of freedom leads to the discussion of Sudakov effects which are crucial for protecting the calculation from dangerous infrared regions. The picture generalizes to many hard reactions; a strategy to extract distribution amplitudes from future data is sketched. We discuss also the particular case of hadron-hadron collisions where the independent scattering mechanism dominates asymptotically and where a different factorization formula applies. We briefly present the concepts of color transparency and nuclear filtering and critically discuss the few present data on this subject.
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ELFE : an Electron Laboratory for Europe
This paper presents a brief overview of the physics with the 15-30~GeV continuous beam electron facility proposed by the European community of nuclear physicists to study the quark and gluon structure of hadrons.
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High Energy Hadronic Helicity Violation: What Helicity Flip Form Factors Measure
Orbital angular momentum is crucial for the description of several process at large momentum transfer in the impulse approximation in perturbative QCD. We review the mechanism of independent scattering which makes violation of the helicity conservation rule possible at leading order - and without any need to flip a quark helicity - in exclusive hadron-hadron reactions. We argue that helicity flip form factors, which violate the factorization assumptions of conventional approaches and are classified as ``higher-twist," are nevertheless calculable to leading order in perturbative QCD when information on non-zero orbital angular momentum is incorporated.
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