Relaxing Constraints from Lepton Flavor Violation in 5D Flavorful Theories

TL;DR

This paper introduces new mechanisms in 5D warped models that relax constraints from lepton flavor violation, allowing for lower KK mass scales compatible with neutrino data and electroweak precision tests.

Contribution

It proposes novel 5D representations and symmetry choices that suppress lepton flavor violation while fitting neutrino data, improving model viability.

Findings

KK scale can be ~3 TeV without violating flavor constraints

New lepton representations enable small charged lepton mixing angles

Enhanced custodial protection reduces Z-mediated flavor violation

Abstract

We propose new mechanisms for ameliorating the constraints on the Kaluza-Klein (KK) mass scale from charged lepton flavor violation in the framework of the Standard Model (SM) fields propagating in a warped extra dimension, especially in models accounting for neutrino data. These mechanisms utilize the extended five-dimensional (5D) electroweak gauge symmetry [SU(2)_L x SU(2)_R x U(1)_X] which is already strongly motivated in order to satisfy electroweak precision tests in this framework. New choices of representations for leptons under this symmetry (naturally) can allow small mixing angles for left-handed (LH) charged leptons and simultaneously large mixing angles for their SU(2)_L partners, i.e., the LH neutrinos, with the neutrino data being accounted for by the latter mixings. Enhancement of charged lepton flavor violation by the large mixing angle observed in leptonic charged currents, which is present for the minimal choice of representations where the LH charged lepton and neutrino mixing angles are similar, can thus be avoided in these models. This idea might also be useful for suppressing the contributions to B_{d, s} mixing in this framework and in order to suppress flavor violation from superpartners (instead of from KK modes) in 5D "flavorful supersymmetry" models. Additionally, the less minimal representations can provide custodial protection for shifts in couplings of fermions to Z and, in turn, further suppress charged lepton flavor-violation from tree-level Z exchange in the warped extra-dimensional scenario. As a result, ~O(3) TeV KK mass scale can be simultaneously consistent with charged lepton flavor violation and neutrino data, even without any particular structure in the 5D flavor parameters in the framework of a warped extra dimension.