# Fermion Mass Hierarchy and Phenomenology in the 5D Domain Wall Standard Model

## TL;DR

This paper proposes a 5D domain wall model to naturally explain the fermion mass hierarchy and explores its phenomenological implications, including constraints from FCNC processes and potential signals at the LHC.

## Contribution

It introduces a novel 5D domain wall setup for the Standard Model that addresses fermion mass hierarchy and analyzes its experimental viability and signatures.

## Key findings

- Fermion localization explains mass hierarchy naturally.
- Heavy KK gauge bosons are consistent with current FCNC constraints.
- Light KK gauge bosons at TeV scale could be detected at the LHC.

## Abstract

We have recently proposed a setup of the "Domain-Wall Standard Model" in 5D spacetime, where all the Standard Model (SM) fields are localized in certain domains of the extra 5th dimension. Utilizing this setup, we attempt to solve the fermion mass hierarchy problem of the SM. The mass hierarchy can be naturally explained by suitably distributing the fermions in different positions along the extra dimension. Due to these different localization points, the effective 4D gauge couplings of Kaluza-Klein (KK) mode gauge bosons to the SM fermions become non-universal. As a result, our model is severely constrained by the Flavor Changing Neutral Current (FCNC) measurements. We find two interesting cases in which our model is phenomenologically viable: (1) the KK-mode of the SM gauge bosons are extremely heavy and unlikely to be produced at the Large Hadron Collider (LHC), while future FCNC measurements can reveal the existence of these heavy modes. (2) the width of the localized SM fermions is very narrow, leading to almost universal 4D KK-mode gauge couplings. In this case, the FCNC constraints can be easily avoided even if a KK gauge boson mass lies at the TeV scale. Such a light KK gauge boson can be searched at the LHC in the near future.