A keV String Axion from High Scale Supersymmetry

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 $1{0}^{11--12}\text{ }\text{ }\mathrm{GeV}$. 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.