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 M-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-susy and S and imposing that M-susy 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, log(10)(m(a)/eV) = -21.5(-2.3)(+1.3). We find that M-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.