Constraints on Models with Warped Extra Dimensions

TL;DR

This paper explores various geometries of warped extra dimensions beyond the common five-dimensional AdS space, analyzing their viability for resolving the gauge hierarchy problem by examining constraints from electroweak and flavor physics.

Contribution

It systematically studies the constraints on Kaluza-Klein scales in diverse warped geometries and dimensions, highlighting challenges and potential advantages of models with bulk Higgs and soft walls.

Findings

Constraints are large in five-dimensional models.

Higher-dimensional models often have dense KK spectra or strong coupling issues.

Bulk Higgs models reduce flavor constraints and suggest a minimum fermion mass.

Abstract

It has been known for some time that warped extra dimensions offer a potential explanation of the large hierarchy that exists between the electroweak scale and the Planck scale. The majority of this work has focused on a five dimensional slice of AdS space. This thesis attempts to address the question, what possible spaces offer phenomenologically viable resolutions to this gauge hierarchy problem. In order for a space to offer a potential resolution to the hierarchy problem two conditions must be met: Firstly one should be able to demonstrate that the space can be stabilised such that a small effective electroweak scale (or large effective Planck scale) can be obtained. Secondly one must demonstrate that the space allows for a Kaluza Klein (KK) scale that is small enough such that one does not reintroduce a hierarchy in the effective theory. Here we focus on the second condition and examine the constraints, on the KK scale, coming from corrections to electroweak observables and flavour physics which arise when gauge fields propagate in an additional dimension. We study a large class of possible spaces of different geometries and dimensionalities. In five dimensions it is found that such constraints are generically large. In more than five dimensions it is found that a significant proportion of such spaces suffer from either a high density of KK modes or alternatively strongly coupled KK fields. The latter would not offer viable resolutions to the hierarchy problem. Models in which the Higgs propagates in the bulk are also studied, in the context of models with a `soft wall' and it is found these have significantly reduced constraints from flavour physics as well as a notion of a minimum fermion mass.