Cosmological window onto the string axiverse and the supersymmetry breaking scale

Abstract

The low-energy limit of several string theories feature the axiverse, a plethora of light axions with masses spanning various orders of magnitude. Within the simplest picture, the masses of the resulting axions depend on a hard non-perturbative scale related to the supersymmetry breaking scale $M_{\rm susy}$, and the dimensionless instanton action $S$ responsible for breaking the axion shift symmetry. We explore whether these two parameters can be probed using cosmology, taking into account current limits on the tensor-to-scalar ratio and the primordial isocurvature fraction, and measurements of the dark matter energy density today. Adopting string-inspired flat priors on $\log_{10}M_{\rm susy}$ and $S$, and imposing that $M_{\rm susy}$ be sub-Planckian, we find $S=198\pm28$ at 68%~C.L., in striking agreement with the theoretical expectation that $S \approx 200$, whereas $M_{\rm susy}$ remains unconstrained.

Luca Visinelli

Assistant Professor of Theoretical Physics (RTT)

Theoretical physicist working across particle physics, astrophysics, cosmology, and experimental searches.