Top Quark Cross-Section Measurements at the Tevatron

Run II of the Tevatron collider at Fermilab is well under way and data samples larger than those of Run I are at hand. In this contribution I summarize the current status of cross-section measurements for top-quark pair production at the CDF and D0 experiments.Comment: 7 pages, 3 figures. To appear in the proceedings of International Europhysics Conference on High-Energy Physics (HEP 2003), Aachen, Germany, 17-23 Jul 200

Explosion phenomena in stochastic coagulation-fragmentation models

First we establish explosion criteria for jump processes with an arbitrary locally compact separable metric state space. Then these results are applied to two stochastic coagulation-fragmentation models--the direct simulation model and the mass flow model. In the pure coagulation case, there is almost sure explosion in the mass flow model for arbitrary homogeneous coagulation kernels with exponent bigger than 1. In the case of pure multiple fragmentation with a continuous size space, explosion occurs in both models provided the total fragmentation rate grows sufficiently fast at zero. However, an example shows that the explosion properties of both models are not equivalent.Comment: Published at http://dx.doi.org/10.1214/105051605000000386 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

Spatial patterns of segregation in a monocentric city - a model with a special production function for housing services

Usually in the monocentric city models, the spatial patterns of segregated ethnic groups are assumed to be ring-shaped, while early in the 1930ies, Homer Hoyt showed, that wedge-shaped areas predominate empirically. After Rose-Ackerman's discussion of the influence of aversion one group of households has against living close to members of another group within a ring-shaped segregated pattern, Yinger showes that a wedge-shaped pattern could arise, depending on the population mix, as long as border length is responsible for the spatial pattern. In this contribution, a monocentric model with the assumption of different household groups, a specific production function for housing and other goods and a specific utility function of households is presented. At first, border length is founded as a criterion of optimality. Secondly, it is shown that mixed patterns of concentric and wedge-shaped areas represent multiple equilibria if more than two groups of households are being considered. The welfare optimal segregated pattern depends on the relative production coefficient of households of different groups in the production of goods. This, for example, allows to analyse the impact of actual global production conditions on the spatial patterns of segregation in cities.

A simulation of segregation in cities and its application for the analysis of price regulation

Social segregation in cities takes place where different household groups exist and when, according to Schelling, their location choice either minimizes the number of differing households in their neighbourhood or maximizes their own group. In this contribution an evolutionary simulation based on a monocentric city model with externalities among households is used to discuss the spatial segregation patterns of four groups. The resulting complex spatial patterns can be shown as graphic animations. They can be applied as initial situation for the analysis of the effects a price regulation has on segregation. JEL classification: D62, R14, R31, R52 Keywords: simulation, segregation, monocentric city, price regulation

Induced gelation in a two-site spatial coagulation model

A two-site spatial coagulation model is considered. Particles of masses $m$ and $n$ at the same site form a new particle of mass $m+n$ at rate $mn$. Independently, particles jump to the other site at a constant rate. The limit (for increasing particle numbers) of this model is expected to be nondeterministic after the gelation time, namely, one or two giant particles randomly jump between the two sites. Moreover, a new effect of induced gelation is observed--the gelation happening at the site with the larger initial number of monomers immediately induces gelation at the other site. Induced gelation is shown to be of logarithmic order. The limiting behavior of the model is derived rigorously up to the gelation time, while the expected post-gelation behavior is illustrated by a numerical simulation.Comment: Published at http://dx.doi.org/10.1214/105051605000000755 in the Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute of Mathematical Statistics (http://www.imstat.org

"Halt, Europol!" : Probleme der europäischen Polizeikooperation für parlamentarische Kontrolle und Grundrechtsschutz

as zusammenwachsende Europa und die Öffnung der Binnengrenzen bringen Touristen und Geschäftsleuten eine Reihe von Erleichterungen. Allerdings profitieren auch Organisierte Kriminalität und transnationaler Terrorismus von dem Verzicht auf Grenzkontrollen zwischen den EU-Staaten. Zur Erhöhung der inneren Sicherheit in einem Europa der offenen Grenzen wurde das Europäische Polizeiamt (Europol) ins Leben gerufen. Zwar hat es keine exekutiven Befugnisse wie die Ausstellung von Haftbefehlen oder Hausdurchsuchungen, denn dies soll weiterhin Aufgabe der nationalen Polizeikräfte sein. Doch Europol sammelt und analysiert Daten zu Straftätern, Verdächtigen, Tatzeugen und Kontaktpersonen im Bereich der grenzüberschreitenden Kriminalität. Damit trägt Europol zu effektiveren Strafverfolgung in den Mitgliedsstaaten bei. Gleichzeitig stellt die Speicherung und Verarbeitung personenbezogener Daten bereits einen Eingriff in das Grundrecht der informationellen Selbstbestimmung dar. Für die nationalen Parlamente wird die Kontrolle der Polizei durch die Europäisierung der Polizeiarbeit erschwert. Zugleich fehlen dem Europäischen Parlament die Kompetenzen, das Kontrolldefizit der nationalen Parlamente auszugleichen. Somit findet die europäische Polizeikooperation weitgehend ohne parlamentarische Beteiligung statt. Eine ähnliche Lücke tut sich im Bereich der gerichtlichen Kontrolle auf. Die Zuständigkeiten der nationalen Gerichte sind eingeschränkt, ohne dass der Europäische Gerichtshof für den Schutz der Grundrechte verantwortlich gemacht worden ist. Der Entwurf für einen europäischen Verfassungsvertrag, der 2002/2003 ausgehandelt wurde, gibt sowohl dem Europäischen Parlament als auch dem Europäischen Gerichtshof dringend benötigte Befugnisse, um die Kontrolle der Kontrolleure zu verbessern. Allerdings droht die Ratifizierung an ganz anderen Streitpunkten – vor allem in Großbritannien – zu scheitern. Für diesen Fall empfiehlt Wolfgang Wagner, die unstrittigen Reformen im Bereich der Polizeikooperation zügig in europäisches Recht umzusetzen