Family Unification via Quasi-Nambu-Goldstone Fermions in String Theory

TL;DR

This paper investigates a string theory-based approach to family unification, utilizing quasi-Nambu-Goldstone fermions arising from symmetry breaking in heterotic string models, aiming to produce realistic chiral matter spectra.

Contribution

It proposes a novel realization of family unification in heterotic string theory through symmetry breaking associated with the spin bundle's orientation change, leading to chiral matter similar to domain-wall fermions.

Findings

Chiral matter fields emerge on defects analogous to domain-wall fermions.

The chiral zero mode spectrum aligns with nonlinear sigma model predictions.

The model connects to orbifold GUT and gauge-Higgs unification frameworks.

Abstract

Some of the supersymmetric nonlinear sigma models on exceptional groups (E7 or E8) are known to yield almost minimal necessary content of matter fields for phenomenological model building, including three chiral families and a Higgs multiplet. We explore a possible realization of such a family unification in heterotic string theory, where the spontaneously broken symmetry that we focus on is the one associated with the change of global orientation of the spin bundle embedded in the gauge bundle, which cannot be ignored in the presence of a magnetic source of the B field. We show in a simple model that it indeed gives rise to chiral matter fields on the defect similarly to the domain-wall fermions, and compare the chiral zero mode spectrum with the sigma model. The setup is reduced to a system similar to that postulated in the orbifold GUT or the grand gauge-Higgs unification model, where the origin of Z2 identification is accounted for as arising from the bolt of the Atiyah-Hitchin manifold.