Suppressing Lepton Flavour Violation in a Soft-Wall Extra Dimension

TL;DR

This paper explores a soft-wall extra dimension model that reduces lepton flavour violation rates, enabling lower Kaluza-Klein scales, but introduces hierarchy challenges for neutrino mass accommodation.

Contribution

It demonstrates that soft-wall models can suppress lepton flavour violation, allowing for lower Kaluza-Klein scales compared to hard-wall models, using a numerical approach for fermion localization.

Findings

Lepton flavour violation rates are significantly reduced in soft-wall models.

Kaluza-Klein scale can be as low as 2 TeV while satisfying constraints.

Introducing small neutrino masses requires a large hierarchy of scales.

Abstract

A soft-wall warped extra dimension allows one to relax the tight constraints imposed by electroweak data in conventional Randall-Sundrum models. We investigate a setup, where the lepton flavour structure of the Standard Model is realised by split fermion locations. Bulk fermions with general locations are not analytically tractable in a soft-wall background, so we follow a numerical approach to perform the Kaluza-Klein reduction. Lepton flavour violation is induced by the exchange of Kaluza-Klein gauge bosons. We find that rates for processes such as muon-electron conversion are significantly reduced compared to hard-wall models, allowing for a Kaluza-Klein scale as low as 2 TeV. Accommodating small neutrino masses forces one to introduce a large hierarchy of scales into the model, making pressing the question of a suitable stabilisation mechanism.