A keV String Axion from High Scale Supersymmetry

TL;DR

This paper proposes that a keV string axion from high scale supersymmetry breaking could explain the 3.5 keV X-ray line observed in galaxy clusters, linking low-energy signals to high-scale physics.

Contribution

It introduces a model where a light string axion from high scale supersymmetry breaking accounts for the X-ray line, connecting cosmological observations with high energy theories.

Findings

A 10 keV axion can produce observable X-ray lines.

High scale supersymmetry is consistent with current limits and explains the axion mass.

The model avoids isocurvature and domain wall problems during inflation.

Abstract

Various theoretical and experimental considerations motivate models with high scale supersymmetry breaking. While such models may be difficult to test in colliders, we propose looking for signatures at much lower energies. We show that a keV line in the X-ray spectrum of galaxy clusters (such as the recently disputed 3.5 keV observation) can have its origin in a universal string axion coupled to a hidden supersymmetry breaking sector. A linear combination of the string axion and an additional axion in the hidden sector remains light, obtaining a mass of order 10 keV through supersymmetry breaking dynamics. In order to explain the X-ray line, the scale of supersymmetry breaking must be about $10^{11-12}$ GeV. This motivates high scale supersymmetry as in pure gravity mediation or minimal split supersymmetry and is consistent with all current limits. Since the axion mass is controlled by a dynamical mass scale, this mass can be much higher during inflation, avoiding isocurvature (and domain wall) problems associated with high scale inflation. In an appendix we present a mechanism for dilaton stabilization that additionally leads to $\mathcal{O}(1)$ modifications of the gaugino mass from anomaly mediation.