Dark gauge sectors and axions are well-motivated in string theory. We demonstrate that if a confining gauge sector gives rise to dark glueballs that are a fraction of the dark matter, and the associated axion has a decay constant near the string scale, then this axion is ultralight and naturally realizes the fuzzy dark matter scenario with a modest tuning of a temperature ratio. Astrophysical observations constrain the size of the glueball component relative to the axionic component, while electric dipole moments constrain mixing with the QCD axion.
@article{arxiv.1805.06011,
title = {Dark Glueballs and their Ultralight Axions},
author = {James Halverson and Brent D. Nelson and Fabian Ruehle and Gustavo Salinas},
journal= {arXiv preprint arXiv:1805.06011},
year = {2018}
}