A relativistic coupled-channel formalism for the pion form factor

The electromagnetic form factor of a confined quark-antiquark pair is calculated within the framework of point-form relativistic quantum mechanics. The dynamics of theexchanged photon is explicitly taken into account by treating theelectromagnetic scattering of an electron by a meson as a relativistic two-channel problem for a Bakamjian-Thomas type mass operator. This approach guarantees Poincare invariance. Using a Feshbach reduction the coupled-channel problem can be converted into a one-channel problem for the elastic electron-meson channel. By comparing the one-photon-exchange optical potential at the constituent and hadronic levels, we are able to unambiguously identify the electromagnetic meson form factor. Violations of cluster-separability properties, which are inherent in the Bakamjian-Thomas approach, become negligible for sufficiently large invariant mass of the electron-meson system. In the limit of an infinitely large invariant mass, an equivalence with form-factor calculations done in front-form relativistic quantum mechanics is established analytically.Comment: 3 pages, 1 figure, submitted to EPJ Web of Conference

Point-form quantum field theory and meson form factors

We shortly review point-form quantum field theory, i.e. the canonical quantization of a relativistic field theory on a Lorentz-invariant surface of the form $x_\mu x^\mu = \tau^2$. As an example of how point-form quantum field theory may enter the framework of relativistic quantum mechanics we discuss the calculation of the electromagnetic form factor of a confined quark-antiquark pair (e.g. the pion).Comment: 3 pages, 2 figures. Based on a talk presented by W. Schweiger at the 20th European Conference on Few-Body Problems in Physics, September 10-14 2007, Pisa, Ital

Dropping cold quantum gases on Earth over long times and large distances

We describe the non-relativistic time evolution of an ultra-cold degenerate quantum gas (bosons/fermions) falling in Earth's gravity during long times (10 sec) and over large distances (100 m). This models a drop tower experiment that is currently performed by the QUANTUS collaboration at ZARM (Bremen, Germany). Starting from the classical mechanics of the drop capsule and a single particle trapped within, we develop the quantum field theoretical description for this experimental situation in an inertial frame, the corotating frame of the Earth, as well as the comoving frame of the drop capsule. Suitable transformations eliminate non-inertial forces, provided all external potentials (trap, gravity) can be approximated with a second order Taylor expansion around the instantaneous trap center. This is an excellent assumption and the harmonic potential theorem applies. As an application, we study the quantum dynamics of a cigar-shaped Bose-Einstein condensate in the Gross-Pitaevskii mean-field approximation. Due to the instantaneous transformation to the rest-frame of the superfluid wave packet, the long-distance drop (100m) can be studied easily on a numerical grid.Comment: 18 pages latex, 5 eps figures, submitte

Revisiting global trends in freshwater insect biodiversity: A reply

Abstract Jähnig et al. make some useful points regarding the conclusions that can be drawn from our meta‐analysis; however, some issues require clarification. First, we never suggested that there was a globally increasing trend of freshwater insect abundances, but only spoke of an average increasing trend in the available data. We also did not suggest that freshwater quality has improved globally, but rather that documented improvements in water quality can explain at least some of the trends we observed. Second, as we acknowledged, our data are not a representative set of freshwater ecosystems around the world, but they are what is currently accessible. Third, there is indeed no doubt that changes in abundance or biomass need not correlate with changes in other aspects of biodiversity, such as species richness or functional composition. Our analysis was specifically focused on trends in community abundance/biomass because it has been the subject of recent study and speculation, and is a widely available metric in long‐term studies. To better understand the recent changes in freshwater insect assemblages, we encourage freshwater ecologists to further open their troves of data from countless long‐term monitoring schemes so that larger and more comprehensive syntheses can be undertaken

Internet Safety: Positioning VCU as a National Leader in Internet Safety

While a multitude of information from a host of sources exists on how to keep children safe on the Internet, there is not a unified effort to combine it all and get it to the right people. This is not a plan to teach college students about Internet safety. This is a proposal to begin much earlier, targeting middle-school aged children and their parents, many of whom have no idea of the dangers – and opportunities – that exist in cyberspace

Form Factors of Few-Body Systems: Point Form Versus Front Form

We present a relativistic point-form approach for the calculation of electroweak form factors of few-body bound states that leads to results which resemble those obtained within the covariant light-front formalism of Carbonell et al. Our starting points are the physical processes in which such form factors are measured, i.e. electron scattering off the bound state, or the semileptonic weak decay of the bound state. These processes are treated by means of a coupled-channel framework for a Bakamjian-Thomas type mass operator. A current with the correct covariance properties is then derived from the pertinent leading-order electroweak scattering or decay amplitude. As it turns out, the electromagnetic current is affected by unphysical contributions which can be traced back to wrong cluster properties inherent in the Bakamjian-Thomas construction. These spurious contributions, however, can be separated uniquely, as in the covariant light-front approach. In this way we end up with form factors which agree with those obtained from the covariant light-front approach. As an example we will present results for electroweak form factors of heavy-light systems and discuss the heavy-quark limit which leads to the famous Isgur-Wise function.Comment: Presented at LIGHTCONE 2011, Dallas, USA, 23 - 27 May, 201