• Randall Sundrum
• extra dimensions
• beyond the standard model

The Standard Model (SM) is a quantum field theory that describes three of the four fundamental interactions known in nature. Despite the success achieved throughout the decades, the theory leaves some phenomena unexplained, particularly it gives rise to the Hierarchy Problem, which occurs when the fundamental value of some physical parameter, such as a coupling constant or a mass, is widely different from its effective measured value. For example, in the SM, there’s no natural mechanism that explains why the weak force is 1024 times stronger than gravity.
A possible solution to this problem is offered by the Randall-Sundrum model, proposed in 1999 by Lisa Randall and Raman Sundrum, which is a 5-dimensional model with a warped geometry and an extra spatial dimension. This model involves a 5-dimensional bulk which contains two branes: the Planck-brane (or UV-brane), where gravity is a relatively strong force, and the TeV-brane (or IR brane) where SM particles live. With the introduction of the so called a warp factor and a not necessarily large distance between the two branes, it’s possible to exponentially suppress the energy scales along the extra dimension, naturally taking the Planck scale down to the energy scale of the Electroweak Symmetry Breaking (∼ O(1) TeV). Naturalness implies that the lightest KK states should then have masses of the order TeV scale.
However CP-violating processes mediated by Kaluza-Klein modes are in excess of experimental bounds unless the IR scale is at least O(10) TeV. Although this bound can be avoided with additional structure (such as flavor symmetries), electroweak precision tests still require an IR scale larger than the electroweak scale, leaving a Little Hierarchy Problem between ∼ O(10) and ∼ O(1) TeV.
In this work I study an extension of this model in which the 5-dimensional bulk is divided in two separate regions by an intermediate (3+1)-brane. The introduction of this defect in the 5D bulk allows us to exploit the fact that different types of fields can propagate different amounts into the bulk, localizing the Higgs field in the intermediate brane. If the energy scale relative to this intermediate brane is ∼ O(10) TeV, the theory describes a Higgs compositeness scenario at this scale, which directly suppresses all virtual KK-mediated electroweak, flavor and CP violating effects enough to be consistent with all precision experiments to date. Furthermore, unlike the standard Randall-Sundrum model, the KK excitations of the gravitational and gauge fields have predominantly flavor-blind and flavor/CP-safe interactions with the Standard Model, escaping the strong constraints from the tests.
In the context of a 5D effective theory, where the structure of the branes is not described explicitly but rather by localized terms, this work aims to investigate the possibility of localizing the KK fields in the UV region of the 5-dimensional bulk, without discussing the dynamics of the UV theory in detail.