Semi-exclusive DVCS

We consider Semi-Exclusive Deeply Virtual Compton Scattering, gamma* p \to gamma Y (SECS), where Y is an inclusive state of intermediate mass, \Lambda_QCD << m_Y << Q. When the photon is produced with a large transverse momentum k_T \sim m_Y the subprocess is hard and the struck quark fragments independently of the target spectators. Using completeness this allows to express the SECS cross section in terms of ordinary parton distributions. Apart from direct comparisons with data (yet to come) new information on Bloom-Gilman duality may be obtained through comparisons of resonance production via DVCS (gamma* p \to gamma N*) with the SECS scaling distribution in m_Y.Comment: 6 pages, 2 figure

Comover Enhancement of Quarkonium Production

Quarkonium data suggest an enhancement of the hadroproduction rate from interactions of the heavy quark pair with a comoving color field generated in the hard gg -> Q\bar{Q} subprocess. We review the motivations and principal consequences of this comover enhancement scenario (CES).Comment: 7 pages, 1 eps figure, to appear in the proceedings of the CERN 2001-2002 workshop `Hard Probes in Heavy Ion Collisions at the LHC

Turbulence, Complexity, and Solar Flares

The issue of predicting solar flares is one of the most fundamental in physics, addressing issues of plasma physics, high-energy physics, and modelling of complex systems. It also poses societal consequences, with our ever-increasing need for accurate space weather forecasts. Solar flares arise naturally as a competition between an input (flux emergence and rearrangement) in the photosphere and an output (electrical current build up and resistive dissipation) in the corona. Although initially localised, this redistribution affects neighbouring regions and an avalanche occurs resulting in large scale eruptions of plasma, particles, and magnetic field. As flares are powered from the stressed field rooted in the photosphere, a study of the photospheric magnetic complexity can be used to both predict activity and understand the physics of the magnetic field. The magnetic energy spectrum and multifractal spectrum are highlighted as two possible approaches to this.Comment: 2 figure

Many-Body Dynamics and Exciton Formation Studied by Time-Resolved Photoluminescence

The dynamics of exciton and electron-hole plasma populations is studied via time-resolved photoluminescence after nonresonant excitation. By comparing the peak emission at the exciton resonance with the emission of the continuum, it is possible to experimentally identify regimes where the emission originates predominantly from exciton and/or plasma populations. The results are supported by a microscopic theory which allows one to extract the fraction of bright excitons as a function of time.Comment: 11 pages, 5 figure

The hbar Expansion in Quantum Field Theory

We show how expansions in powers of Planck's constant hbar = h/2\pi can give new insights into perturbative and nonperturbative properties of quantum field theories. Since hbar is a fundamental parameter, exact Lorentz invariance and gauge invariance are maintained at each order of the expansion. The physics of the hbar expansion depends on the scheme; i.e., different expansions are obtained depending on which quantities (momenta, couplings and masses) are assumed to be independent of hbar. We show that if the coupling and mass parameters appearing in the Lagrangian density are taken to be independent of hbar, then each loop in perturbation theory brings a factor of hbar. In the case of quantum electrodynamics, this scheme implies that the classical charge e, as well as the fine structure constant are linear in hbar. The connection between the number of loops and factors of hbar is more subtle for bound states since the binding energies and bound-state momenta themselves scale with hbar. The hbar expansion allows one to identify equal-time relativistic bound states in QED and QCD which are of lowest order in hbar and transform dynamically under Lorentz boosts. The possibility to use retarded propagators at the Born level gives valence-like wave-functions which implicitly describe the sea constituents of the bound states normally present in its Fock state representation.Comment: 8 pages, 1 figure. Version to be published in Phys. Rev.

Polarization as a Probe to the Production Mechanisms of Charmonium in πN\pi N Collisions

Measurements of the polarization of \jp produced in pion-nucleus collisions are in disagreement with leading twist QCD prediction where \jp is observed to have negligible polarization whereas theory predicts substantial polarization. We argue that this discrepancy cannot be due to poorly known structure functions nor the relative production rates of \jp and $\chi_J$. The disagreement between theory and experiment suggests important higher twist corrections, as has earlier been surmised from the anomalous non-factorized nuclear $A$-dependence of the \jp cross section.Comment: 8 page