Exclusive electroproduction and off-diagonal parton distributions

Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale.Comment: 7 pages, 3 figures, LaTeX2e. Talk given at the Fourth Workshop on Quantum Chromodynamics, Paris, France, 1-6 June 199

Inverse scattering theory and trace formulae for one-dimensional Schr\"odinger problems with singular potentials

Inverse scattering theory is extended to one-dimensional Schr\"odinger problems with near-boundary singularities of the form $v(z\to 0)\simeq -z^{-2}/4+v_{-1}z^{-1}$. Trace formulae relating the boundary value $v_0$ of the nonsingular part of the potential to spectral data are derived. Their potential is illustrated by applying them to a number of Schr\"odinger problems with singular potentials.Comment: 12 pages, no figures, pdf-LaTeX; v3: published version, 24 pages, uses IOP style files, minor changes, introduction extended, references adde

Polarisation in deeply virtual meson production

We discuss two aspects of polarisation in hard exclusive meson production: the leading-twist selection rule for the meson helicity, and the different partial waves of a (pi pi)-pair which may or may not be due to the decay of a rho.Comment: 9 pages, 1 figure. Talk given at the Workshop on Exclusive and Semiexclusive Processes at High Momentum Transfer, Jefferson Lab, May 1999. Remark added on the transition from two gluons to a pion pai

Timelike Compton scattering: exclusive photoproduction of lepton pairs

We investigate the exclusive photoproduction of a heavy timelike photon which decays into a lepton pair, gamma p -> l+ l- p. This can be seen as the analog of deeply virtual Compton scattering, and we argue that the two processes are complementary for studying generalized parton distributions in the nucleon. In an unpolarized experiment the angular distribution of the leptons readily provides access to the real part of the Compton amplitude. We estimate the possible size of this effect in kinematics where the Compton process should be dominated by quark exchange.Comment: 31 pages, 17 figure

Hard exclusive reactions and hadron structure

The generalized Bjorken regime of exclusive reactions opens new ways to explore the hadron structure. We shortly review the present status of this domain where generalized parton distributions, generalized distribution amplitudes and transition distribution amplitudes describe various aspects of confinement physics.Comment: 7 pages LATEX, 5 figures, invited talk at the Vth Conference on Hadronic Physics held at ICTP, Trieste, Italy, May 200

Identifying Prognostic Indicators for Electrical Treeing in Solid Insulation through PD Analysis

This paper presents early results from an experimental study of electrical treeing on commercially available pre-formed silicone samples. A needle-plane test arrangement was set up using hypodermic needles. Partial discharge (PD) data was captured using both the IEC 60270 electrical method and radio frequency (RF) sensors, and visual observations are made using a digital microscope. Features of the PD plot that corresponded to electrical tree growth were assessed, evaluating the similarities and differences of both PD measurement techniques. Three univariate phase distributions were extracted from the partial discharge phase-resolved (PRPD) plot and the first four statistical moments were determined. The implications for automated lifetime prediction of insulation samples due to electrical tree development are discussed

QCD factorizations in gamma* gamma* -> rho rho

We calculate the lowest order QCD amplitude, i.e. the quark exchange contribution, to the forward production amplitude of a pair of longitudinally polarized $\rho$ mesons in the scattering of two virtual photons $\gamma^*(Q_1) \gamma^*(Q_2) \to \rho^0_L \rho^0_L$. We show that the scattering amplitude simultaneously factorizes in two quite different ways: the part with transverse photons is described by the QCD factorization formula involving the generalized distribution amplitude of two final $\rho$ mesons, whereas the part with longitudinally polarized photons takes the QCD factorized form with the $\gamma^*_L \to \rho^0_L$ transition distribution amplitude. Perturbative expressions for these, in general, non-perturbative functions are obtained in terms of the $\rho-$meson distribution amplitude

Exclusive production of pentaquarks in the scaling regime

We investigate two exclusive reactions with a Theta+ pentaquark in the final state: electroproduction of a K meson on the nucleon, and K+ scattering on a neutron target producing a lepton pair. These reactions offer unique opportunities to investigate the structure of pentaquark baryons at parton level. We discuss the generalized parton distributions for the N --> Theta+ transition and give the leading order amplitude for these processes in the Bjorken regime.Comment: 5 pages in LATEX, 1 .eps figure, talk at the Baryons04 Conference, October 25-29, 2004, Palaiseau, Franc