Standard Model Naturalness from Dark Vortices and Codimension-2 Braneworlds

Abstract

There are numerous models of new physics that posit extra dimensions with characteristic length scale much larger than the inverse TeV scale. These so-called models of large extra dimensions can be used to confront the hierarchy problems that plague the standard models of particle physics and cosmology, as the usual arguments about hierarchy problems can be evaded if there are new physics scales and new dynamics in the extra dimensions. This thesis investigates a class of 6D models where the size of the two extra dimensions is exponentially sensitive to the value of a bulk zero mode, allowing a large hierarchy of scales to be generated if the zero mode is stabilized at a modestly large value. In general, scale invariance in the bulk forces the zero mode potential to have a runaway (or flat) form, so localized brane sources are added to the system which explicitly break the scale invariance and stabilize the zero mode. Brane physics can be chosen so that this stabilization naturally happens at values that give micron-sized extra dimensions, as desired in models of large extra dimensions that solve the electroweak hierarchy problem. These models are also interesting because they can predict a 4D curvature that is suppressed relative to the mass scale of the brane physics, thereby making progress on the cosmological constant problem by separating the 4D particle physics scale from the scale of the observed 4D vaccuum energy. This suppression is technically natural because the curvature vanishes in the limit that the branes are scale invariant, though this is also the limit in which the runaway potential for the zero mode reappears. This thesis also constructs the correct 4D effective theory that describes the system when length scales of interest are much larger than the size of the extra dimensions, and an effective field theory approach is also adopted to study the Higgs phenomenology of these models at higher energies.