Flavor Singlet Axial Coupling of the Proton - An Updated Analysis

We present a combined analysis of SESAM and TxL data for the flavor singlet axial coupling G_A^1 of the proton, which is very helpful to stabilize the disconnected signals at small quark masses. From connected and disconnected contributions we use the tadpole improved renormalization constant Z_A and obtain G_A^1=0.21(12).Comment: 3 pages, contribution to LATTICE99 (matrix elements

A Polarized HERA Collider

A brief review is given of the status of nucleon spin structure functions as determined from polarized deep inelastic lepton-nucleon scatttering, including current outstanding problems. The characteristics of a polarized HERA collider, some of the particle physics topics it could address, and the accelerator physics challenges it must meet are discussed.Comment: 5 Pages, 6 Figures, To be published in the Proceedings of the DIS99 Conference April 1999, DESY Zeuthen, German

Quenching of hadron spectra in DIS on nuclear targets

The multiple scatterings incurred by a hard quark produced in a nuclear medium induce the emission of soft gluons which carry a fraction of the quark energy and eventually affect the hadronization process. Here, the depletion of semi-inclusive hadron spectra in DIS on various nuclei (N, Ne, Cu, Kr) is computed as a function of nu and z to leading order in alpha_s through medium-modified fragmentation functions. Using the transport coefficient q previously determined from Drell-Yan production, the predictions are found to be in good agreement with EMC and HERMES preliminary data. Calculations on Xe targets are also presented and discussed.Comment: 20 pages, 8 figure

Hadron Formation in DIS in a nuclear environment

The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep inelastic scattering has been studied by the HERMES experiment at DESY using 27.5 GeV positrons. A substantial reduction of the multiplicity of charged hadrons and identified charged pions from nuclei relative to that from deuterium has been measured as function of the relevant kinematic variables. The preliminary results on krypton show a larger reduction of the multiplicity ratio $R_M^{h}$ with respect to the one previously measured on nitrogen and suggest a possible modification of the quark fragmentation process in the nuclear environment.Comment: 5 pages, 5 figures, Proceeding of DIS 2001, IX International Workshop on Deep Inelastic Scattering Bologna, 27 April-1 May 200

The nature and value of cultural policy research for early career researchers.

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Polarized lepton nucleon scattering - summary of the experimental spin sessions at DIS 99

This paper summarizes the contributions to the experimental sessions on polarized lepton nucleon scattering at the DIS 99 workshop. Results are reported about the flavor decomposition of the quark polarization, a first direct measurement of a positive gluon polarization, the observation of a double-spin asymmetry in diffractive rho production, the polarization of lambda hyperons, the observation of transverse single-spin asymmetries and the measurement of the Gerasimov-Drell-Hearn sum rule. Prospects of future fixed target and collider facilities are discussed.Comment: 8 pages, 5 figures, Proc. of the 7th Int. Workshop on Deep Inelastic Scattering and QCD (DIS99), Zeuthen, Germany, April 19-23, 1999, to appear in Nucl. Phys. B (Proc. Suppl.

High-energy exclusive leptoproduction of vector mesons

The physics of diffractive vector meson production in virtual photon nucleon scattering at NMC energies is reviewed. A particular attention is paid to the physical aspects of the reaction and how they influence the observables. The reaction is a good probe to investigate both soft exchange mechanisms and hadronic wave functions. Extension to either HERA or ELFE kinematics is sketched out.Comment: Contribution to the workshop "ELFE at DESY" (St-Malo, Sep. 96), 15 pages, LaTeX file, 8 figures, uses eps

On the Q^2 Dependence of Measured Polarized Structure Funtions

We analyse the available data on the polarized asymmetries $A_1$ for proton, neutron and deuteron targets. We use a homogeneous and updated set of unpolarized structure functions to derive $g_1$ from $A_1$, and we accurately correct for the scaling violations in order to obtain $g_1(x,Q^2)$ with the same $Q^2$ for all $x$ values. The contribution to the $Q^2$ evolution of a possible large gluon polarized density is also considered. The implications for the Ellis-Jaffe and for the Bjorken sum rules are discussed.Comment: 11 pages (LaTex) + 4 figures (topdrawer, included at the end) CERN-TH 7023/9

Polarized Structure Functions in the Valence Quark and Resonance Regions and the GDH Sum

I present in this paper the neutron spin physics program in Hall A at Jefferson Laboratory using a polarized helium-3 target. The program encompasses several completed experiments, in which, valuable spin observables (spin dependent structure functions) were measured in order to learn about how the nucleon spin arises from the behavior of the constituents. These experiments also offer a ground for testing our understanding of the strong regime of quantum chromodynamics (QCD) the theory of strong interactions through the determination of moments of these structure functions.Comment: 9 pages, 2 Postscript figures, XVth International Conference on Particles and Nuclei (PANIC02), Osaka, Japan, 30 September-4 October 200

Parton distributions in a constituent quark scenario

A simple picture of the constituent quark as a composite system of point-like partons is used to construct the unpolarized and polarized parton distributions by a convolution between constituent quark momentum distributions and constituent quark structure functions. We achieve good agreement with experiments in the unpolarized, as well as, in the polarized case. When our results are compared with similar calculations using non-composite constituent quarks, the accord with the experiments of the present scheme is impressive. We conclude that DIS data are consistent with a low energy scenario dominated by composite constituents of the nucleon.Comment: 4 pages; latex using espcrc1.sty; 4 postscript figures; Invited talk at the Workshop ``Nucleon '99'', Frascati; Italy 7-9 June 1999. Submitted to Nuc. Phys.