Cosmological window onto the string axiverse and the supersymmetry breaking scale

In the simplest picture, the masses of string axions populating the axiverse depend on two parameters: the supersymmetry-breaking scale Msusy 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 log₁₀Msusy and S and imposing that Msusy be sub-Planckian, we find S=198±28. These bounds suggest that cosmological data complemented with string-inspired priors select a quite narrow axion mass range within the axiverse, log₁₀(mₐ/eV)=-21.5_-2.3+1.3. We find that Msusy 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.