Development and Evaluation of a Simulation System of Electric-Powered Wheelchairs for Training Purposes

Não consta

Quiver theories, soliton spectra and Picard-Lefschetz transformations

Quiver theories arising on D3-branes at orbifold and del Pezzo singularities are studied using mirror symmetry. We show that the quivers for the orbifold theories are given by the soliton spectrum of massive 2d N=2 theory with weighted projective spaces as target. For the theories obtained from the del Pezzo singularities we show that the geometry of the mirror manifold gives quiver theories related to each other by Picard-Lefschetz transformations, a subset of which are simple Seiberg duals. We also address how one indeed derives Seiberg duality on the matter content from such geometrical transitions and how one could go beyond and obtain certain ``fractional Seiberg duals.'' Moreover, from the mirror geometry for the del Pezzos arise certain Diophantine equations which classify all quivers related by Picard-Lefschetz. Some of these Diophantine equations can also be obtained from the classification results of Cecotti-Vafa for the 2d N=2 theories.Comment: 34 pages, 11 figure

A Vector Non-abelian Chern-Simons Duality

Abelian Chern-Simons gauge theory is known to possess a `$S$-self-dual' action where its coupling constant $k$ is inverted {\it i.e.} $k \leftrightarrow {1 \over k}$. Here a vector non-abelian duality is found in the pure non-abelian Chern-Simons action at the classical level. The dimensional reduction of the dual Chern-Simons action to two-dimensions constitutes a dual Wess-Zumino-Witten action already given in the literature.Comment: 14+1 pages, LaTeX file, no figures, version to appear in Phys. Rev

The FLASHForward Facility at DESY

The FLASHForward project at DESY is a pioneering plasma-wakefield acceleration experiment that aims to produce, in a few centimetres of ionised hydrogen, beams with energy of order GeV that are of quality sufficient to be used in a free-electron laser. The plasma wave will be driven by high-current density electron beams from the FLASH linear accelerator and will explore both external and internal witness-beam injection techniques. The plasma is created by ionising a gas in a gas cell with a multi-TW laser system, which can also be used to provide optical diagnostics of the plasma and electron beams due to the <30 fs synchronisation between the laser and the driving electron beam. The operation parameters of the experiment are discussed, as well as the scientific program.Comment: 19 pages, 9 figure

Non-abelian T-duality, Ramond Fields and Coset Geometries

We extend previous work on non-abelian T-duality in the presence of Ramond fluxes to cases in which the duality group acts with isotropy such as in backgrounds containing coset spaces. In the process we generate new supergravity solutions related to D-brane configurations and to standard supergravity compactifications.Comment: 35 pages, Late

Classical Inhomogeneities in String Cosmology

We generalize previous work on inhomogeneous pre-big bang cosmology by including the effect of non-trivial moduli and antisymmetric-tensor/axion fields. The general quasi-homogeneous asymptotic solution---as one approaches the big bang singularity from perturbative initial data---is given and its range of validity is discussed, allowing us to give a general quantitative estimate of the amount of inflation obtained during the perturbative pre-big bang era. The question of determining early-time ``attractors'' for generic pre-big bang cosmologies is also addressed, and a motivated conjecture is advanced. We also discuss S-duality-related features of the solutions, and speculate on the way an asymptotic T-duality symmetry may act on moduli space as one approaches the big bang.Comment: 32 pages, Revtex, 1 figure included, epsfig.sty needed; one reference adde

Quantum equivalence of sigma models related by non Abelian Duality Transformations

Coupling constant renormalization is investigated in 2 dimensional sigma models related by non Abelian duality transformations. In this respect it is shown that in the one loop order of perturbation theory the duals of a one parameter family of models, interpolating between the SU(2) principal model and the O(3) sigma model, exhibit the same behaviour as the original models. For the O(3) model also the two loop equivalence is investigated, and is found to be broken just like in the already known example of the principal model.Comment: As a result of the collaboration of new authors the previously overlooked gauge contribution is inserted into eq.(43) changing not so much the formulae as part of the conclusion: for the models considered non Abelian duality is OK in one loo

Possible Origin of Fermion Chirality and Gut Structure From Extra Dimensions

The fundamental chiral nature of the observed quarks and leptons and the emergence of the gauge group itself are most puzzling aspects of the standard model. Starting from general considerations of topological properties of gauge field configurations in higher space-time dimensions, it is shown that the existence of non-trivial structures in ten dimensions would determine a class of models corresponding to a grand unified GUT structure with complex fermion representations with respect to $SU(3)_C \otimes SU(2)_L \otimes U(1)_Y$. The discussion is carried out within the framework of string theories with characteristic energy scales below the Planck mass. Avoidance of topological obstructions upon continuous deformation of field configurations leads to global chiral symmetry breaking of the underlying fundamental theory, imposes rigorous restrictions on the structure of the vacuum and space-time itself and determines uniquely the gauge structure and matter content.Comment: final version to appear in Phys. Rev.