Cosmological window onto the string axiverse and the supersymmetry breaking scale

TL;DR

This paper investigates how cosmological data can constrain string axion parameters, revealing a narrow axion mass range and a specific instanton action, but leaving the supersymmetry breaking scale unconstrained due to parameter degeneracy.

Contribution

It introduces a method to use cosmological data with string-inspired priors to constrain axion parameters, highlighting the narrow axion mass range and the degeneracy with the supersymmetry breaking scale.

Findings

String instanton action S = 198 ± 28

Narrow axion mass range log10(m_a/eV) = -21.5^{+1.3}_{-2.3}

Supersymmetry breaking scale M_susy remains unconstrained

Abstract

In the simplest picture, the masses of string axions populating the axiverse depend on two parameters: the supersymmetry breaking scale $M_{\rm susy}$ 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 $\log_{10}M_{\rm susy}$ and $S$, and imposing that $M_{\rm susy}$ be sub-Planckian, we find $S=198\pm28$. These bounds suggest that cosmological data complemented with string-inspired priors select a quite narrow axion mass range within the axiverse, $\log_{10}(m_a/{\rm eV}) = -21.5^{+1.3}_{-2.3}$. We find that $M_{\rm susy}$ 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.