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Abstract

This thesis investigates phenomenological implications of flux compactifications in type IIB string theory. In particular, we focus on the phenomena of gauge kinetic mixing and cosmic inflation in large volume scenarios in type IIB.

In the first part we discuss kinetic mixing and study settings with U(1)s from sequestered D-brane sectors focusing on mixing of D3-D3 and D7-D7-branes. Strikingly, kinetic mixing is absent due to a non-trivial cancellation in rather generic scenarios. Specifically for D3-branes, precise calculations of string diagrams have previously demonstrated a cancellation on toroidal geometries.Also, field theoretic 10d supergravity approaches have shown that a cancellation remains at leading order between the contributions of B2 and C2 in the case of D3-brane mixing, extending the result from special geometries to realistic Calabi-Yau settings. We take the latter approach and furthermore consider non-zero values for C0 and include sub-leading terms of the D3-brane action and affirm that an exact cancellation persists in this generalised setting. Ultimately, we demonstrate that this cancellation is tied to the SL(2,R) self-duality of type IIB. Finally, allowing for SL(2,R)-breaking 3-form fluxes, kinetic mixing between D3-branes arises at a volume-suppressed level. In the case of D7-D7-brane mixing, we consider stacks of D7-branes where the non-abelian gauge theory is broken by internal flux of the gauge theory. In this case we find that, in addition to B2 and C2 contributions, non-vanishing kinetic mixing is induced by C4. Yet again, the B2 and C2 contributions cancel if no 3-form fluxes are present. We derive explicit formulas for kinetic mixing in both cases and perform a phenomenological analysis of our D3-D3 scenario and find that parametrically small values of kinetic mixing can be realised.

In the second part we discuss cosmic inflation in string theory. We consider a scenario of type IIB where all moduli are stabilised realising a large volume and a Minkowski minimum. The volume and an additional small blow-up modulus are fixed by non-perturbative effects, while the other \Kahler moduli are stabilised via loop corrections. Within this framework, we investigate different regimes of the scalar potential which are suitable for slow roll and show that flat plateaus exist quite generally. We apply our observation to a concrete and simple model where we use an additional blowup mode as the inflaton. In this model the respective inflationary potential becomes flat for large field values. We ensure that our model aligns with the observed normalization of scalar perturbations and generates an adequate number of e-foldings. As a consequence, the volume cannot be stabilised at excessively high values and the inflaton starts rolling at largish values, thus introducing a control issue for the stabilisation procedure. Nevertheless, we demonstrate that our model remains in a controlled regime. Encouragingly, a thorough analysis indicates that our model meets all the necessary phenomenological criteria.