Open string axions and the flavor problem

TL;DR

This paper explores open string axions in D-brane inspired models, addressing flavor textures and anomaly cancellation, and examines their compatibility with astrophysical constraints and the flavor problem.

Contribution

It introduces models with open string axions that can explain flavor textures and satisfy astrophysical bounds, highlighting the rarity of minimal models without fine-tuning.

Findings

Open string axions are common in D-brane models but often conflict with bounds.

Some models achieve acceptable axion decay constants within allowed ranges.

Minimal models free from excessive fine-tuning are identified.

Abstract

We consider extensions of the standard model inspired by intersecting D-brane constructions, in order to address flavor mass textures. We include additional anomalous gauge symmetries, and scalar fields to break them and to generate Froggatt-Nielsen mass terms. Green-Schwarz axions are included to cancel mixed anomalies rendering the models consistent. At low energies, a residual anomalous global symmetry remains, and its associated pseudo-Goldstone mode becomes the physical axion, which can be interpreted as an axion arising from open string modes. General considerations show that such axions are very common in D-brane models and can be completely incompatible with current bounds. Astrophysical constraints are placed on the axion both by including neutrino masses in the Froggatt-Nielsen scheme and considering QCD instanton contributions to the axion mass. We find simple models where the axion decay constant is in the allowed range, but only one such minimal model with this property is free from excessive fine tunings elsewhere. We also note that generically addressing flavor textures for the CKM matrix leads to deconstructed extra dimensions.