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

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

Competition of Heavy Quark Radiative and Collisional Energy Loss in Deconfined Matter

We extend our recently advanced model on collisional energy loss of heavy quarks in a quark gluon plasma (QGP) by including radiative energy loss. We discuss the approach and present first preliminary results. We show that present data on nuclear modification factor of non photonic single electrons hardly permit to distinguish between those 2 energy loss mechanisms.Comment: 8 pages, extended to 11 pages for v2; accepted for publication in Journal of Physics

Non-universality of transverse Coulomb exchange at small x

Within an explicit scalar QED model we compare, at fixed x << 1, the leading-twist K_T-dependent `quark' distribution f_q(x, K_T) probed in deep inelastic scattering and Drell-Yan production, and show that the model is consistent with the universality of f_q(x, K_T). The extension of the model from the aligned-jet to the 'symmetric' kinematical regime reveals interesting properties of the physics of Coulomb rescatterings when comparing DIS and DY processes. At small x the transverse momentum induced by multiple scattering on a single centre is process dependent, as well as the transverse momentum broadening occurring in collisions on a finite size nuclear target.Comment: 28 pages, 3 eps figure

Radio emission from cosmic ray air showers: simulation results and parametrization

We have developed a sophisticated model of the radio emission from extensive air showers in the scheme of coherent geosynchrotron radiation, providing a theoretical foundation for the interpretation of experimental data from current and future experiments. Having verified the model through comparison of analytic calculations, Monte Carlo simulations and historical experimental data, we now present the results of extensive simulations performed with our Monte Carlo code. Important results are the absence of significant asymmetries in the total field strength emission pattern, the spectral dependence of the radiation, the polarization characteristics of the emission (allowing an unambiguous test of the geomagnetic emission mechanism), and the dependence of the radio emission on important air shower and observer parameters such as the shower zenith angle, the primary particle energy, the depth of the shower maximum and the observer position. An analytic parametrization incorporating the aforementioned dependences summarizes our results in a particularly useful way.Comment: 33 pages, 24 figures, final version as accepted for publication by Astropart. Physics, only minor updates since V

Deeply Virtual Compton Scattering: How to Test Handbag Dominance?

We propose detailed tests of the handbag approximation in exclusive deeply virtual Compton scattering. Those tests make no use of any prejudice about parton correlations in the proton which are basically unknown objects and beyond the scope of perturbative QCD. Since important information on the proton substructure can be gained in the regime of light cone dominance we consider that such a class of tests is of special relevance