In the simplest picture, the masses of string axions populating the axiverse depend on two parameters: the supersymmetry-breaking scale $M_{\mathrm{susy}}$ and the action $S$ of the string instantons responsible for breaking the axion shift symmetry. In this work, we explore whether cosmological data can be used to probe these two parameters. Adopting string-inspired flat priors on ${\log }_{10}{M}_{\mathrm{susy}}$ and $S$ and imposing that $M_{\mathrm{susy}}$ be sub-Planckian, we find $S=198\pm 28$. These bounds suggest that cosmological data complemented with string-inspired priors select a quite narrow axion mass range within the axiverse, ${\log }_{10}(m_a/\mathrm{eV})=-21.5_{-2.3}^{+1.3}$. We find that $M_{\mathrm{susy}}$ remains unconstrained due to a fundamental parameter degeneracy with $S$. We explore the significant impact of other choices of priors on the results, and we comment on similar findings in recent previous literature.

• Received 20 September 2018

DOI:https://doi.org/10.1103/PhysRevD.99.063517

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society