Vector mesons in a relativistic point-form approach

We apply the point form of relativistic quantum mechanics to develop a Poincare invariant coupled-channel formalism for two-particle systems interacting via one-particle exchange. This approach takes the exchange particle explicitly into account and leads to a generalized eigenvalue equation for the Bakamjian-Thomas type mass operator of the system. The coupling of the exchange particle is derived from quantum field theory. As an illustrative example we consider vector mesons within the chiral constituent quark model in which the hyperfine interaction between the confined quark-antiquark pair is generated by Goldstone-boson exchange. We study the effect of retardation in the Goldstone-boson exchange by comparing with the commonly used instantaneous approximation. As a nice physical feature we find that the problem of a too large $\rho$-$\omega$ splitting can nearly be avoided by taking the dynamics of the exchange meson explicitly into account.Comment: 14 pages, 1 figur

Soil respiration in native Cerrado and a pasture in Central Brazil: Seasonal variation and deep soil contribution on total flux.

Soil respiration is perhaps one of the most important in the many different processes of the carbon cycling in natural and cultivated ecosystems. The aim of this study was to measure intensively the soil respiration fluxes in two native Cerrado vegetation types, that differ in tree density (cerrado denso and cerrado stricto sensu) and one planted pasture, measuring total annual fluxes and their seasonal variation. The effect of environmental variables (temperature and rainfall) and the role of deep soil on superficial emissions were also assessed. The native Cerrado ecosystems are located in the Reserva Ecológica do Roncador (RECOR/IBGE), in Brasília, DF, Brazil, while the planted pasture (Brachiaria brizantha) is located in the experimental fields of Embrapa-Cerrados, in Planaltina, DF, Brazil. Soil respiration was measured with the dynamic chamber IRGA technique. Deep soil fluxes were estimated from soil atmosphere samples taken at different depths, and the CO2 flux on each soil layer was calculated using a gas diffusivity model. The total annual emissions were greater in the pasture, with sharper seasonal variation when compared to the native ecosystems, which were very similar. For all ecosystems, fluxes were closely correlated with soil temperature, and with cumulative precipitation in different timescales. In all three ecosystems (for several occasions), we found a significant CO2 production in the deeper layers of the soil, especially in the depth of 75-100 cm

Internal carbon cycling in native Cerrado and a pasture in Central Brazil.

During the past decade, the increasing knowledge about the effects of natural and cultivated ecosystems on regional and global climate raised the importance of terrestrial ecosystem to function either as carbon source or sink. The aim of this study was to measure the most important pools and fluxes of the internal carbon cycling in two native Cerrado vegetation types, that differ in tree density (cerrado denso and cerrado stricto sensu) and one planted pasture, in an effort to produce a synthesis of our knowledge about Cerrado carbon cycling. The native Cerrado ecosystems are located in the Reserva Ecológica do Roncador (RECOR/IBGE), in Brasília, DF, Brazil, while the planted pasture (Brachiaria brizantha) is located in the experimental fields of Embrapa-Cerrados, in Planaltina, DF, Brazil.. Synthesis of estimations of the main pools and fluxes were made by combining direct field measurements, data from the literature , and modeling results. The native ecosystems showed very similar pools and fluxes. Pools in living tissues of pasture were much smaller than natural ecosystems, while soil organic matter was slightly larger in the pasture. The fluxes were faster in the pasture. The intensity of fluxes intensity in the pasture, makes the carbon dynamics of this ecosystem very sensitive to management practices

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

Multiplicity, Invariants and Tensor Product Decomposition of Tame Representations of U(\infty)

The structure of r-fold tensor products of irreducible tame representations of the inductive limit U(\infty) of unitary groups U(n) are are described, versions of contragredient representations and invariants are realized on Bargmann-Segal-Fock spaces.Comment: 48 pages, LaTeX file, to appear in J. Math. Phy

Divisive Normalization and Neuronal Oscillations in a Single Hierarchical Framework of Selective Visual Attention

Divisive normalization models of covert attention commonly use spike rate modulations as indicators of the effect of top-down attention. In addition, an increasing number of studies have shown that top-down attention increases the synchronization of neuronal oscillations as well, particularly in gamma-band frequencies (25–100 Hz). Although modulations of spike rate and synchronous oscillations are not mutually exclusive as mechanisms of attention, there has thus far been little effort to integrate these concepts into a single framework of attention. Here, we aim to provide such a unified framework by expanding the normalization model of attention with a multi-level hierarchical structure and a time dimension; allowing the simulation of a recently reported backward progression of attentional effects along the visual cortical hierarchy. A simple cascade of normalization models simulating different cortical areas is shown to cause signal degradation and a loss of stimulus discriminability over time. To negate this degradation and ensure stable neuronal stimulus representations, we incorporate a kind of oscillatory phase entrainment into our model that has previously been proposed as the “communication-through-coherence” (CTC) hypothesis. Our analysis shows that divisive normalization and oscillation models can complement each other in a unified account of the neural mechanisms of selective visual attention. The resulting hierarchical normalization and oscillation (HNO) model reproduces several additional spatial and temporal aspects of attentional modulation and predicts a latency effect on neuronal responses as a result of cued attention

The experiences of children with disabilities and primary caregivers on the social inclusion of children with disabilities in Ethiopia

This article presents the findings of a qualitative study on the social inclusion of children with disabilities in Addis Ababa, Ethiopia and how this affects their achievement of valued life goals. The study is based on focus group discussions with children with disabilities and primary caregivers of such children. The thematic analysis is based on family quality of life and the capability approach, which are used to explain the effects of social inclusion (and exclusion) on children with disabilities and their families in three areas of quality of life: being, belonging, and becoming. The study also looked at how the social inclusion of children with disabilities can be actualised. The results of the study confirm the existence of considerable challenges to the social inclusion of children with disabilities in Addis Ababa, which is reflected in their capabilities. The results suggest that children with disabilities need assistance and support to achieve a good quality of life. They also indicate the need for the involvement of the wider community to support children with disabilities and their families to enhance the capability of such children, and so improve their family quality of life