A Heterotic QCD Axion

Understand

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.