905 research outputs found
Realisations of Symmetry
We perform a systematic investigation of free-scalar realisations of the
Za\-mo\-lod\-chi\-kov algebra in which the operator product of two
spin-three generators contains a non-zero operator of spin four which has
vanishing norm. This generalises earlier work where such an operator was
required to be absent. By allowing this spin-four null operator we obtain
several realisations of the algebra both in terms of two scalars as well
as in terms of an arbitrary number of free scalars. Our analysis is
complete for the case of two-scalar realisations.Comment: 14 pages, LATEX, UG-6/9
Intersecting Branes in Matrix Theory
We construct BPS states in the matrix description of M-theory. Starting from
a set of basic M-theory branes, we study pair intersections which preserve
supersymmetry. The fractions of the maximal supersymmetry obtained in this way
are 1/2, 1/4, 1/8, 3/16 and 1/16. In explicit examples we establish that the
matrix BPS states correspond to (intersecting) brane configurations that are
obtained from the d=11 supersymmetry algebra. This correspondence for the 1/2
supersymmetric branes includes the precise relations between the charges.Comment: 11 pages, LaTeX, no figures, minor changes, shortened version to be
published in Physics Letters
Atomically Precise Nanocrystals
Nanocrystals are a state-of-matter in the border area between molecules and bulk materials. Unlike bulk materials, nanocrystals have size-dependent properties, yet the question remains whether nanocrystal properties can be analyzed, understood, and controlled with atomic precision, a key characteristic of molecules. Acknowledging the inclination of nanocrystals to form defect structures, we first outline the prospects of atomically precise analysis. A broad spectrum of analytical methods has become available over the last five years, such that for heterogeneous nanocrystal ensembles, a single, atomically precise representative structure can be determined to explore structure-property relations. Atomically precise synthesis, on the other hand, remains an outstanding challenge that may well face fundamental limitations. However, to amplify properties and prepare nanocrystals for specific applications, full atomic precision may not be needed. Examples of an atomic precision light approach, focusing on exact thickness or facet control, exist and can inspire scientists to explore atomic precision in nanocrystal research further
The European Flood Alert System – Part 1: Concept and development
International audienceThis paper presents the development of the European Flood Alert System (EFAS), which aims at increasing preparedness for floods in trans-national European river basins by providing local water authorities with medium-range and probabilistic flood forecasting information 3 to 10 days in advance. The EFAS research project started in 2003 with the development of a prototype at the European Commission Joint Research Centre (JRC), in close collaboration with the national hydrological and meteorological services. The prototype covers the whole of Europe on a 5 km grid. In parallel, different high-resolution data sets have been collected for the Elbe and Danube river basins, allowing the potential of the system under optimum conditions and on a higher resolution, to be assessed. Flood warning lead-times of 3–10 days are achieved through the incorporation of medium-range weather forecasts from the Deutscher Wetterdienst (DWD) and the European Centre for Medium-Range Weather Forecasts (ECMWF), comprising a full set of 51 probabilistic forecasts from the Ensemble Prediction System (EPS) provided by ECMWF. The ensemble of different hydrographs is analysed and combined to produce early flood warning information, which is disseminated to the hydrological services that have agreed to participate in the development of the system. In Part 1 of this paper, the scientific approach adopted in development of the system is presented. The rational of the project, the system's set-up, its underlying components, basic principles, and products, are described. In Part 2, results of a detailed statistical analysis of the performance of the system are shown, with regard to both probabilistic and deterministic forecast
Scaling Cosmologies of N=8 Gauged Supergravity
We construct exact cosmological scaling solutions in N=8 gauged supergravity.
We restrict to solutions for which the scalar fields trace out geodesic curves
on the scalar manifold. Under these restrictions it is shown that the axionic
scalars are necessarily constant. The potential is then a sum of exponentials
and has a very specific form that allows for scaling solutions. The scaling
solutions describe eternal accelerating and decelerating power-law universes,
which are all unstable. An uplift of the solutions to 11-dimensional
supergravity is carried out and the resulting timedependent geometries are
discussed. In the discussion we briefly comment on the fact that N=2 gauged
supergravity allows stable scaling solutions.Comment: 17 pages; referenced added, reportnr changed and some corrections in
section
Kirche im Dienst der Publizistik
Die hier folgenden Gedanken spiegeln die Erfahrung eines kanadischen Bischofswider. Er lebt inmitten einer pluralistischen Gesellschaft in einem zweisprachigenLand, in dem die Massenkommunikationsmittel größtenteils kommerziellen Rücksichten unterliegen. Meine Überlegungen stimmen in der Linie mit dem Zweiten Vatikanum überein. Das Konzil rief uns ja u. a. folgende Wahrheiten ins Gedächtnis: Wir sind ein Pilgervolk und haben keine bleibende Heimat hier. (...)EnglishThe Canadian Bishop de Roo in his speech at the UNDA General Assembly in New Orleans asks if the church as well as putting the means of social communication at her "service" should not "serve" the world of social communications. This "service" would mean e. g. to recognize the real nature of the social communications media in order to respect their true potential; to accept dialogue; to recognize the signs of the times and discern in them the presence of god; to listen to what the world is saying to us through the media; to accept the various expressions of truth without wishing to channel them only along the familiar lines of a changing culture
Superpotentials in IIA compactifications with general fluxes
We derive the effective N=1, D=4 supergravity for the seven main moduli of
type IIA orientifolds with D6 branes, compactified on T^6/(Z_2xZ_2) in the
presence of general fluxes. We illustrate and apply a general method that
relates the N=1 effective Kahler potential and superpotential to a consistent
truncation of gauged N=4 supergravity. We identify the correspondence between
various admissible fluxes, N=4 gaugings and N=1 superpotential terms. We
construct explicit examples with different features: in particular, new IIA
no-scale models and a model which admits a supersymmetric AdS_4 vacuum with all
seven main moduli stabilized.Comment: 29 pages, published versio
Potential and mass-matrix in gauged N=4 supergravity
We discuss the potential and mass-matrix of gauged N=4 matter coupled
supergravity for the case of six matter multiplets, extending previous work by
considering the dependence on all scalars. We consider all semi-simple gauge
groups and analyse the potential and its first and second derivatives in the
origin of the scalar manifold. Although we find in a number of cases an
extremum with a positive cosmological constant, these are not stable under
fluctuations of all scalar fields.Comment: 28 pages, LaTe
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