Leptons in Holographic Composite Higgs Models with Non-Abelian Discrete Symmetries

TL;DR

This paper explores holographic composite Higgs models incorporating non-Abelian discrete symmetries to naturally explain neutrino masses and mixing, analyzing two specific 5D models that satisfy current experimental bounds.

Contribution

It introduces two novel 5D gauge-Higgs unification models with non-Abelian discrete symmetries for leptons, addressing neutrino mass generation and flavor violation constraints.

Findings

Models accommodate normal and inverted neutrino mass orderings.

Both models comply with current Lepton Flavour Violation bounds.

Majorana model allows KK scale around 3.5 TeV, Dirac model requires >10 TeV.

Abstract

We study leptons in holographic composite Higgs models, namely in models possibly admitting a weakly coupled description in terms of five-dimensional (5D) theories. We introduce two scenarios leading to Majorana or Dirac neutrinos, based on the non-abelian discrete group $S_4\times \Z_3$ which is responsible for nearly tri-bimaximal lepton mixing. The smallness of neutrino masses is naturally explained and normal/inverted mass ordering can be accommodated. We analyze two specific 5D gauge-Higgs unification models in warped space as concrete examples of our framework. Both models pass the current bounds on Lepton Flavour Violation (LFV) processes. We pay special attention to the effect of so called boundary kinetic terms that are the dominant source of LFV. The model with Majorana neutrinos is compatible with a Kaluza-Klein vector mass scale $m_{KK}\gtrsim 3.5$ TeV, which is roughly the lowest scale allowed by electroweak considerations. The model with Dirac neutrinos, although not considerably constrained by LFV processes and data on lepton mixing, suffers from a too large deviation of the neutrino coupling to the $Z$ boson from its Standard Model value, pushing $m_{KK}\gtrsim 10$ TeV.