Pair production of stop quarks in e + e - [e hoch + e hoch -] annihilation close to threshold

Wir studieren die Stop-Antistop Paarproduktion in Elektron-Positron Kollisionen, wenn die invariante Masse des Eingangszustandes in der Nähe der Schwellenenergie liegt. Mit v bezeichnen wir die Relativgeschwindigkeit der Squarks im Schwerpunktsystem. Die Masse der Stop Teilchen m, ihr räumlicher Impuls p ~ m v und ihre kinetische Energie ~ m v^2 sind Skalen welche in diesem Problem auftreten. Bei der Berechnung des Wirkungsquerschnittes in der Nähe der Produktionsschwelle (v<<1) müssen Terme der Form ~(alphaS/v)^n$ sowie große Logarithmen von Quotienten der drei oben erwähnten Skalen resummiert werden. Um das zu erreichen wenden wir die Effektive Quantenfeldtheorie vNRQCD an. Die Zerfallsbreite der Stops stellt eine zusätzliche Skala dar, welche wir als O(m v^2) zählen. Aufgrund der Instabilität des Stops kommt es in der effektiven Theorie zu Phasenraumdivergenzen, welche durch das Einführen von Phasenraumcuts auf die Zerfallsprodukte der Stops vermieden werden können. Ähnliche Effekte treten auch bei der Top-Antitop Paarproduktion auf. Bei der Stop-Antistop Paarproduktion treten Phasenraumdivergenzen allerding bereits auf führender Ordnung auf. Auch der Beitrag von Hintergrunddiagrammen ist relativ zur führenden Ordnung gesehen größer als im Falle der Top-Antitop Paarproduktion.We study the cross section for stop-antistop pair production in the electron-positron collision close to threshold for unstable squarks. Scales appearing in this process are the mass of the stop m, their 3-momentum ~ m v and their kinetic energy ~ m v^2, where v is the relative velocity of the squarks in the center of mass frame. Close to production threshold (v<<1) we need to resum terms ~ (alphaS/v)^n as well as large logarithms of ratios of the physical scales in quantum corrections to the cross section. To achieve this we employ the scalar version of the effective field theory framework vNRQCD. The finite width of the stop, which we count as O(m v^2), is an additional scale in the problem. Stop instability effects generate divergences in the phase space integrals and to deal with these in the effective theory kinematic cuts on the final states are introduced. Phase space divergences already enter at leading order and also background diagrams are substantially more important than for top-antitop pair production

An anisotropic effective model for the simulation of semiflexible ring polymers

This is an open access article published under a Creative Commons Attribution (CC-BY) License.We derive and introduce anisotropic effective pair potentials to coarse-grain solutions of semiflexible ring polymers of various lengths. The system has been recently investigated by means of full monomer-resolved computer simulations, revealing a host of unusual features and structure formation, which, however, cannot be captured by a rotationally averaged effective pair potential between the rings' centers of mass [ Bernabei, M.; Soft Matter 2013, 9, 1287 ]. Our new coarse-graining strategy is to picture each ring as a soft, penetrable disk. We demonstrate that for the short- and intermediate-length rings the new model is quite capable of capturing the physics in a quantitative fashion, whereas for the largest rings, which resemble flexible ones, it fails at high densities. Our work opens the way for the physical justification of general, anisotropic penetrable interaction potentials.This work has been supported by the Austrian Science Fund (FWF), Grant 23400-N16.Peer Reviewe

Anisotropic effective interactions and stack formation in mixtures of semiflexible ring polymers

By means of extensive computer simulations, we investigate the formation of columnar structures (stacks) in concentrated solutions of semiflexible ring polymers. To characterize the stacks we employ an algorithm that identifies tube-like structures in the simulation cell. Stacks are found both in the real system and in the fluid of soft disks interacting through the effective anisotropic pair potential derived for the rings [P. Poier et al., Macromolecules, 2015, 48, 4983–4997]. Furthermore, we investigate binary mixtures of cluster-forming and non-cluster-forming rings. We find that monodispersity is not a requirement for stack formation. The latter is found for a broad range of mixture compositions, though the columns in the mixtures exhibit important differences to those observed in the monodisperse case. We extend the anisotropic effective model to mixtures. We show that it correctly predicts stack formation and constitutes a significant improvement with respect to the usual isotropic effective description based only on macromolecular centers-of-mass.This work has been supported by the Austrian Science Fund (FWF), Grant 23400-N16. We acknowledge allocation of CPU time in CSUC (Spain). This work was financially supported by the Austrian Science Fund (FWF) within the project F41 (SFB ViCoM).Peer reviewe

Contrainte et création

Quand il s agit de création humaine et non divine, artistique et non théologique, dire que la création a ses contraintes sonne presque comme une évidence. Tout artiste, qu’il soit écrivain, poète, mais aussi plasticien, musicien, homme ou femme de théâtre, de cinéma, etc. et à quelque siècle qu’il appartienne, connaît ce qu’on appelle trivialement, en amont, les affres de la création et, en aval, l’angoisse de la réception. Le cahier des charges de la création apparaît sans fin. Cependant, comment penser, ainsi que le suggère le titre de ce volume, non seulement la création contrainte, mais surtout la contrainte créatrice ? Venue du dehors ou du dedans, formelle ou thématique, imposée ou librement adoptée, comment la contrainte stimule-t-elle la force créatrice ? Comment l’oriente- t-elle ? Pour répondre à ces questions, les auteurs de cet ouvrage ont souhaité proposer un parcours riche et diversifié, transséculaire, au gré des perspectives et des disciplines. S’appuyant sur l’analyse d’œuvres littéraires et artistiques concrètes, ils ont cherché à saisir dans la facture même de l’œuvre les secrets du rapport mystérieux entre création et contrainte