Future Measurements of Deeply Virtual Compton Scattering at HERMES

Prospects for future measurements of Deeply Virtual Compton Scattering at HERMES are studied using different simple models for parameterizations of generalized parton distributions (GPDs). Measurements of the lepton charge and lepton beam helicity asymmetry will yield important input for theoretical models towards the future extraction of GPDs.Comment: 12 pages, 7 figure

On a Possibility to Determine the Sign of the Polarized Gluon Distribution

We investigate the possibility to draw conclusions on the sign of the spin-dependent gluon distribution, $\Delta G(x, Q^2)$, from existing polarized DIS data. The spin-dependent parton distributions $\Delta u_v, \Delta d_v, \Delta {\bar u}, \Delta {\bar d}, \Delta {s}$, and $\Delta G$ are constructed in the framework of a phenomenological procedure taking into account some assumptions on signs of valence and sea parton distributions motivated by 't Hooft's mechanism of quark-quark interaction induced by instantons. The axial gluon anomaly and data on integral quark contributions to the proton spin, $\Delta \tilde u, \Delta \tilde d$, and $\Delta \tilde s$, are also taken into account. Predictions for the $x$- and $Q^2$-dependencies of the polarized proton and neutron structure functions, $g_1^p$ and $g_1^n$, are compared to experimental data. It is shown that the neutron structure function, $g_1^n$, is especially sensitive to the sign of $\Delta G(x, Q^2)$. The results of our analysis supports the conclusion that this sign should be positive.Comment: 14 pages, latex, 12 figure

Relativistic Diskoseismology

We will summarize results of calculations of the modes of oscillation trapped within the inner region of accretion disks by the strong-field gravitational properties of a black hole (or a compact, weakly-magnetized neutron star). Their driving and damping will also be addressed. The focus will be on the most observable class: the analogue of internal gravity modes in stars. Their frequencies which corrrespond to the lowest mode numbers depend almost entirely upon only the mass and angular momentum of the black hole. Such a feature may have been detected in the X-ray power spectra of two galactic `microquasars', allowing the angular momentum of the black hole to be determined in one case.Comment: To be published in Physics Reports, proceedings of the conference Astrophysical Fluids: From Atomic Nuclei to Stars and Galaxies; 10 pages, 5 postscript figure

Large N_c confinement and turbulence

We suggest that the transition that occurs at large $N_c$ in the eigenvalue distribution of a Wilson loop may have a turbulent origin. We arrived at this conclusion by studying the complex-valued inviscid Burgers-Hopf equation that corresponds to the Makeenko-Migdal loop equation, and we demonstrate the appearance of a shock in the spectral flow of the Wilson loop eigenvalues. This picture supplements that of the Durhuus-Olesen transition with a particular realization of disorder. The critical behavior at the formation of the shock allows us to infer exponents that have been measured recently in lattice simulations by Narayanan and Neuberger in $d=2$ and $d=3$. Our analysis leads us to speculate that the universal behavior observed in these lattice simulations might be a generic feature of confinement, also in $d=4$ Yang-Mills theory.Comment: 4 pages, no figures- Some rewriting - Typos corrected - References completed and some correcte

Transversity Distribution and Polarized Fragmentation Function from Semi-inclusive Pion Electroproduction

A method is discussed to determine the hitherto unknown u-quark transversity distribution from a planned HERMES measurement of a single-spin asymmetry in semi-inclusive pion electroproduction off a transversely polarized target. Assuming u-quark dominance, the measurement yields the shapes of the transversity distribution and of the ratio of a polarized and the unpolarized u-quark fragmentation functions. The unknown relative normalization can be obtained by identifying the transversity distribution with the well-known helicity distribution at large x. The systematic uncertainty of the method is dominated by the assumption of u-quark dominance.Comment: 5 pages, 5 figures, revised version as will be published in EPJ