ETD

• brane geometry
• string theory
• supersymmetry breaking

In this thesis, we investigate how the breaking or absence altogether of supersymmetry affects the geometry of branes in string theory. In particular, we focus on three tachyon-free non-supersymmetric string theories, namely the heterotic SO(16)xSO(16) string and the two orientifolds with gauge groups USp(32) and U(32). In this context, supersymmetry breaking translates into the presence of a dilaton tadpole potential, i.e. a runaway exponential potential for the dilaton, in the low-energy dynamics. In Chapters 2 and 3, we provide a brief introduction to String Theory and discuss the three non-supersymmetric models. Chapter 4 develops the low-energy effective action and the corresponding equations of motion, specialized to an ansatz possessing the symmetries of a brane. Chapters 5 and 6 collect the original work of this thesis: in Chapter 5, we investigate the asymptotic geometry of the D5-brane, refining the description of the pinch-off singularity. We also describe the geometries for uncharged, non-extremal D8- and D7-branes. In Chapter 6 we study extremal near-horizon geometries in the presence of a dilaton tadpole, starting from a supersymmetric near-horizon solution. We construct the near-horizon geometries for the F1-string and, again, for the D5-brane. We then compute the static potential for a probe brane in the new geometries. Chapter 7 collects some concluding remarks and addresses some open questions that could be investigated in the future.