[en] In this thesis, we have discussed various aspects of branes in string theory and M-theory. In chapter 2 we were able to construct six-dimensional chiral interacting eld theories from Hanany-Witten like brane setups. The field theory requirement that the anomalies cancel was reproduced by RR-charge conservation in the brane setup. The data of the Hanany-Witten setup, which consists of brane positions, was mapped to instanton data. The orbifold construction can be extended to D and E type singularities. In chapter 3 we discussed a matrix conjecture, which claims that M-theory in the light cone gauge is described by the quantum mechanics of D0 branes. Toroidal compactifications of M-theory have a description in terms of super Yang-Mills theory an the dual torus. For more than three compactified dimensions, more degrees of freedom have to be added. In some sense, the philosophy in this chapter is orthogonal to the previous chapter: Here, we want to get M-theory results from eld theory considerations, whereas in the previous chapter we obtained eld theory results by embedding the theories in string theory. Our main focus was on the compactification on T⁶, which leads to complications. Here, the Matrix model is again given by an eleven dimensional theory, not by a lower dimensional field theory. Other problems and possible resolutions of Matrix theory are discussed at the end of chapter 3. In the last chapter we considered M- and F-theory compactifications on Calabi-Yau fourfolds. After explaining some basics of fourfolds, we showed that the web of fourfolds is connected by singular transitions. The two manifolds which are connected by the transition are different resolutions of the same singular manifold. The resolution of the singularities can lead to a certain type of divisors, which lead to non-perturbative superpotentials, when branes wrap them. The vacua connected by the transitions can be physically very different. (orig.)

[en] We study the Gepner model description of D-branes in Calabi-Yau manifolds with singular curves. From a geometrical point of view, the resolution of singularities leads to additional homology cycles around which branes can wrap. Using techniques from conformal field theory we address the construction of boundary states for branes wrapping additional 3-cycles on the resolved Calabi-Yau manifold. Explicit formulas are provided for $/BZ₂$ singular curves. (author)

[en] A large class of D-branes in Calabi-Yau spaces can be constructed at the Gepner points using the techniques of boundary conformal field theory. In this note we develop methods that allow to compute open string amplitudes for such D-branes. In particular, we present explicit formulas for the products of open string vertex operators of untwisted A-type branes. As an application we show that the boundary theories of the quintic associated with the special Lagrangian submanifolds Im ω_i z_i = 0$ where ω_i⁵=1 possess no continuous moduli. (author)

[en] We present how to construct 6d fixed point theories via Hanany-Witten setups. It is shown that consistency of the brane setup is equivalent to anomaly freedom of the gauge theory. The brane realization of the fixed point theory allows us to study deformations and phase transitions. Coupling the 6d theory as a flavor system to a 4d Hanany-Witten setup these 6d transitions lead to a chirality changing phase transition in the resulting N = 1 supersymmetric 4d theory. (author)

[en] We review the description of D-branes in Landau Ginzburg models using Matrix Factorizations. A number of simple examples are discussed, the main example being Matrix factorizations for the K3 given by the quartic hypersurface in P₃. We discuss deformations of matrix factorizations and their obstructions in simple cases. Based on Brunner et al., J. High Energy Phys. 0606, 015 (2006) [arXiv:hep-th/0603196]. (Abstract Copyright [2007], Wiley Periodicals, Inc.)

[en] We reconstruct non-trivial 6d theories obtained by Blum and Intriligator by considering IIB or SO(32) 5 branes at ALE spaces in the language of Hanany Witten setups. Using ST duality we make the equivalence of the two approaches manifest, thereby uncovering several new T-duality relations between the group theoretic data describing the embedding of the instantonic 5 brane in the ALE and brane positions in the Hanany Witten language. We construct several new 6d theories, which can be understood as arising on 5 branes in IIB orientifolds with oppositely charged orientifold planes recently introduced by Witten. (author)

[en] In this note we discuss D-branes on T⁴/Z₂ using the boundary states formalism. Explicit formulas for the untwisted boundary states inherited from the underlying T⁴ and twisted states corresponding to branes wrapping collapsed 2-cycles at the orbifold singularities are given. The exact CFT description of the orbifold makes it possible to study how the boundary states transform under R_i → 1/R_i transformation on all directions of the underlying T⁴. We compare their transformation law with results obtained from world volume considerations. (author)

[en] This paper describes the production of induced radioactivity in samples of aluminium, iron and copper exposed to high-energy hadronic radiation. The samples were irradiated in a stray radiation field generated by the interaction of 24 GeV/c protons in the beam dump of the PS IRRAD2 facility ai CERN. The specific radioactivity induced in the samples was measured by gamma spectrometry. The Monte Carlo particle transport code FLUKA was used to simulate the irradiation experiment in order to predict the production of radioactive isotopes in the samples. These predictions are compared with the experimental measurements. (author)

[en] We study D-branes on the quintic CY by combining results from several directions: general results on holomorphic curves and vector bundles, stringy geometry and mirror symmetry, and the boundary states in Gepner models recently constructed by Recknagel and Schomerus, to begin sketching a picture of D-branes in the stringy regime. We also make first steps towards computing superpotentials on the D-brane world-volumes. (author)

[en] We consider the entanglement entropy for the 2D Ising model at the conformal fixed point in the presence of interfaces. More precisely, we investigate the situation where the two subsystems are separated by a defect line that preserves conformal invariance. Using the replica trick, we compute the entanglement entropy between the two subsystems. We observe that the entropy, just like in the case without defects, shows a logarithmic scaling behavior with respect to the size of the system. Here, the prefactor of the logarithm depends on the strength of the defect encoded in the transmission coefficient. We also comment on the supersymmetric case.