5D UED: Flat and Flavorless

TL;DR

This paper investigates flavor violation issues in 5D Universal Extra Dimensions, exploring localization strategies in flat extra dimensions, and finds that flavor constraints are difficult to resolve without mimicking warped geometries, revealing intrinsic tensions in model building.

Contribution

It demonstrates the challenges of using localization in flat extra dimensions to address flavor constraints while preserving KK-parity, and presents solutions to ultra-light mode problems in such models.

Findings

Flavor asymmetric counter-terms are needed on branes.

Localization in flat extra dimensions cannot easily suppress flavor violation.

Ultra-light modes are linked to tension between KK-parity and localization.

Abstract

5D UED is not automatically minimally flavor violating. This is due to flavor asymmetric counter-terms required on the branes. Additionally, there are likely to be higher dimensional operators which directly contribute to flavor observables. We document a mostly unsuccessful attempt at utilizing localization in a flat extra dimension to resolve these flavor constraints while maintaining KK-parity as a good quantum number. It is unsuccessful insofar as we seem to be forced to add brane operators in such a way as to precisely mimic the effects of a double throat warped extra dimension. In the course of our efforts, we encounter and present solutions to a problem common to many extra dimensional models in which fields are "doubly localized:" ultra-light modes. Under scrutiny, this issue seems tied to an intrinsic tension between maintaining Kaluza-Klein parity and resolving mass hierarchies via localization.