Gauge Instantons from Open Strings

In this contribution we describe how to obtain instanton effects in four dimensional gauge theories by computing string scattering amplitudes in D3/D(-1) brane systems. In particular we show that the disks with mixed boundary conditions, which are typical of the D3/D(-1) system, are the sources for the classical instanton solution.Comment: 7 pages, 2 figures, Contribution to the proceedings of the 36th International Symposium Ahrenshoop, Berlin, Germany, 26-30 August 200

Coulomb-gas approach for boundary conformal field theory

We present a construction of boundary states based on the Coulomb-gas formalism of Dotsenko and Fateev. It is shown that Neumann-like coherent states on the charged bosonic Fock space provide a set of boundary states with consistent modular properties. Such coherent states are characterised by the boundary charges, which are related to the number of bulk screening operators through the charge neutrality condition. We illustrate this using the Ising model as an example, and show that all of its known consistent boundary states are reproduced in our formalism. This method applies to $c<1$ minimal conformal theories and provides an unified computational tool for studying boundary states of such theories.Comment: 10 pages, 1 figure, revtex. Minor corrections, references added. To appear in Nuclear Physics

Consumer Advisory Bulletin, Some Guidance on Giving Avoiding charity fraud, October 2011

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Dirichlet Branes on Orbifolds

The D-brane spectrum of a class of \Zop_2 orbifolds of toroidally compactified Type IIA and Type IIB string theory is analysed systematically. The corresponding K-theory groups are determined and complete agreement is found. The charge densities of the various branes are also calculated

The Spacetime Life of a Non-BPS D-particle

We investigate the classical geometry generated by a stable non-BPS D-particle. We consider the boundary state of a stable non-BPS D-particle in the covariant formalism in the type IIB theory orbifolded by (-1)^{F_L} I_4. We calculate the scattering amplitude between two D-particles in the non-compact and compact orbifold and analyse the short and long distance behaviour. At short distances we find no force at order $v^2$ for any radius, and at the critical radius we find a BPS-like behaviour up to $v^4$ corrections for long and short distances. Projecting the boundary state on the massless states of the orbifold closed string spectrum we obtain the large distance behaviour of the classical solution describing this non-BPS D-particle in the non-compact and compact cases. By using the non-BPS D-particle as a probe of the background geometry of another non-BPS D-particle, we recover the no-force condition at the critical radius and the $v^2$ behaviour of the probe. Moreover, assuming that the no-force persists for the complete geometry we derive part of the classical solution for the non-BPS D-particle.Comment: 36 pages, LaTeX, no figures; amplitude for moving particles and analysis of short and long distance interactions included, some assumptions clarified, references adde

Isolde Schaad, Vom Einen. Literatur und Geschlecht. Elf Porträts aus der Gefahrenzone. 2004

Vom Einen. Oder nicht eigentlich von Zweien? Isolde Schaad zitiert Hannah Arendt: "Mann und Frau können [...] nur gleich menschlich sein, indem sie voneinander absolut verschieden sind." Das besage, "was die Frauenbewegung anstrebt" (S. 100). Wenn Mann und Frau absolut verschieden sind, wissen sie nichts voneinander, auch nichts von ihrer Verschiedenheit. Um sie zu erfahren, müssen Mann und Frau sie aufheben, sie in eine Bezugnahme verwandeln, deren Form von ihrem Inhalt verneint und verworfen wird. Ihre gemeinsame Menschlichkeit, in der allein Mann und Frau sich als Mann und Frau wahrnehmen und begreifen können, liegt also in einer Übereinkunft, deren Scheitern ihre Notwendigkeit immer von neuem bestätigt und hervorruf

Non-extremal fractional branes

We construct non-extremal fractional D-brane solutions of type-II string theory at the Z_2 orbifold point of K3. These solutions generalize known extremal fractional-brane solutions and provide further insights into N=2 supersymmetric gauge theories and dual descriptions thereof. In particular, we find that for these solutions the horizon radius cannot exceed the non-extremal enhancon radius. As a consequence, we conclude that a system of non-extremal fractional branes cannot develop into a black brane. This conclusion is in agreement with known dual descriptions of the system.Comment: 29 pages, LaTeX. v2: 30 pages; equation (3.4) corrected; typos fixed; discussion in section 3 streamlined and slightly extended; reference adde

N=2 Instanton Calculus In Closed String Background

In this contribution we describe how to obtain instanton effects in four dimensional gauge theories by computing string scattering amplitudes in D3/D(-1) brane systems. In particular we study a system of fractional D3/D(-1) branes in a Z_2 orbifold and in a Ramond-Ramond closed string background, and show that it describes the gauge instantons of N=2 super Yang-Mills theory and their interactions with the graviphoton of N=2. Using string theory methods we compute the prepotential of the effective gauge theory exploiting the localization methods of the instanton calculus, showing that this leads to the same information given by the topological string.Comment: 12 pages, 2 figures; to appear in the proceedings of 21st Nishinomiya-Yukawa Memorial Symposium on Theoretical Physics: Noncommutative Geometry and Quantum Spacetime in Physics, Nishinomiya and Kyoto, Japan, 11-15 Nov 200

Topological First-Order Systems with Landau-Ginzburg Interactions

We consider the realization of N=2 superconformal models in terms of free first-order $(b,c,\beta,\gamma)$-systems, and show that an arbitrary Landau-Ginzburg interaction with quasi-homogeneous potential can be introduced without spoiling the (2,2)-superconformal invariance. We discuss the topological twisting and the renormalization group properties of these theories, and compare them to the conventional topological Landau-Ginzburg models. We show that in our formulation the parameters multiplying deformation terms in the potential are flat coordinates. After properly bosonizing the first-order systems, we are able to make explicit calculations of topological correlation functions as power series in these flat coordinates by using standard Coulomb gas techniques. We retrieve known results for the minimal models and for the torus.Comment: 37 page