Quark and Lepton Mass and Mixing with non-universal $Z'$ from a 5d Standard Model with gauged $SO(3)$

TL;DR

This paper presents a 5d Standard Model extension with gauged $SO(3)$, generating hierarchical fermion masses and non-universal $Z'$ couplings, explaining neutrino mixing and potential anomalies in semi-leptonic B decays.

Contribution

It introduces a novel 5d model with $SO(3)$ gauging and orbifold symmetry breaking, linking fermion mass hierarchies and $Z'$ couplings to extra-dimensional dynamics.

Findings

Hierarchical fermion masses from 5d Dirac mass hierarchy.

Neutrino mixing via a new type Ib seesaw mechanism.

Non-universal $Z'$ couplings can explain B decay anomalies.

Abstract

We propose a theory of quark and lepton mass and mixing with non-universal $Z'$ couplings based on a 5d Standard Model with quarks and leptons transforming as triplets under a new gauged $SO(3)$ isospin. In the 4d effective theory, the $SO(3)$ isospin is broken to $U(1)'$, through a $S^1/(\mathbb{Z}_2\times \mathbb{Z}'_2)$ orbifold, then subsequently dynamically broken, resulting in a massive $Z'$. Quarks and leptons in the 5d bulk appear as massless modes, with zero Yukawa couplings to the Higgs on the brane, and zero couplings to $Z'$, at leading order, due to the $U(1)'$ symmetry. However, after the $U(1)'$ breaking, both Yukawa couplings and non-universal $Z'$ couplings are generated by heavy Kaluza-Klein exchanges. Hierarchical quark and lepton masses result from a hierarchy of 5d Dirac fermion masses. Neutrino mass and mixing arises from a novel type Ib seesaw mechanism, mediated by Kaluza-Klein Dirac neutrinos. The non-universal $Z'$ couplings may contribute to semi-leptonic $B$ decay ratios which violate $\mu - e$ universality. In this model such couplings are related to the corresponding quark and lepton effective Yukawa couplings.