title: Fundamental and Phenomenological Aspects of Anti-D-Brane Supersymmetry Breaking
Creators: name: Tonioni, Flavio
Publisher: name: University of Liverpool Repository
Year: 2022

description: This dissertation focusses on the investigation of both the fundamental and the phenomenological features of non-supersymmetric string theories based on supersymmetry breaking by anti-D-branes. The study of non-supersymmetric string theories is shedding light on an important corner of the string landscape that might ultimately explain the reason why, so far, supersymmetry has not been detected in the observed universe. The first line of research aims at enriching the understanding of misaligned supersymmetry in String Theory. Misaligned supersymmetry consists in cancellations between bosonic and fermionic contributions at different energy levels in the whole string spectrum. This is interpreted as a physical mechanism that helps visualising the origin of the finiteness of string constructions, otherwise motivated based on the behaviour of the theory under modular transformations. A review is presented of how misaligned supersymmetry in closed-string theories leads to a cancellation between bosons and fermions even in non-supersymmetric scenarios. Then, it is shown that an entirely analogous cancellation can take place in non-supersymmetric open-string theories, too, by studying anti-D-branes placed on top of O-planes. These ideas are then developed via a systematic analysis of the net physical degeneracies at each energy level, studying their non-trivial cancellations and relating them to the modular properties of the partition function. Eventually, the whole concept of misaligned supersymmetry in String Theory is analysed in a mathematically rigorous way, showing the details of how the boson-fermion cancellations can take place in physical quantities, and the role of misalignment in all known 10-dimensional tachyon-free non-supersymmetric string constructions is finally discussed. The second line of research, instead, is devoted to a phenomenological investigation and description of a class of quasi-realistic non-supersymmetric vacua including anti-D-branes. In particular, it discusses model-building scenarios featuring intersecting anti-D3- and D7-branes. Effectively, supersymmetry is broken spontaneously, despite having no scale at which sparticles appear and standard supersymmetry is restored. If the branes are placed on singularities at the tip of warped throats in Calabi-Yau orientifold flux compactifications, they may give rise to quasi-realistic particle spectra, closed- and open-string moduli stabilisation with a Minkowski/de Sitter uplift, together with a geometrical origin for the scale hierarchies. A derivation is given of the low-energy effective field theory description for such scenarios, i.e. a non-linear supergravity theory for standard and constrained supermultiplets, including soft supersymmetry-breaking matter couplings. The effect of closed-string moduli stabilisation on the open-string matter sector is worked out, incorporating non-perturbative and perturbative effects, and the mass and coupling hierarchies are computed with a view towards phenomenology.