A Heterotic QCD Axion

TL;DR

This paper demonstrates how certain heterotic string models can produce a QCD axion with a decay constant in the allowed range by exploiting moduli-dependent Fayet-Iliopoulos terms, avoiding previous no-go constraints.

Contribution

It introduces a heterotic string model framework that yields a phenomenologically viable QCD axion with a lower decay constant through moduli-dependent FI terms.

Findings

Constructed an explicit heterotic line bundle model with a viable axion

Showed that moduli-dependent FI terms can lower the axion decay constant

Avoided previous no-go theorems by using non-universal FI terms

Abstract

We show that a KSVZ axion with a decay constant in the phenomenologically allowed range can be obtained in certain $E_8\times E_8$ heterotic string models. These models have an enhanced symmetry locus in the moduli space, and a non-universal, Kahler moduli dependent Fayet-Iliopoulos term which vanishes at this locus. Close to this locus the Fayet-Iliopoulos term is small and can lead to an axion decay constant significantly lower than the string scale. In this way, the no-go arguments of Svrcek and Witten, which are based on a universal, dilaton-dependent Fayet-Iliopoulos term, can be avoided. The relevant axion originates from phases of bundle moduli which correspond to deformations away from the enhanced symmetry locus. We construct an explicit example, based on a heterotic line bundle standard model, with all the required ingredients.