The 3.5 keV Line from Stringy Axions

TL;DR

This paper presents a string theory-based microscopic model explaining the 3.5 keV X-ray line via decaying 7 keV dark matter into axions that convert into photons in galaxy cluster magnetic fields.

Contribution

It provides a detailed realization of a dark matter decay and axion-photon conversion model within type IIB string compactifications, connecting particle physics to astrophysical observations.

Findings

Constructed a string theory model with a 7 keV dark matter candidate.

Demonstrated how ultra-light axions couple to photons via kinetic mixing.

Analyzed moduli stabilization and coupling strengths in the model.

Abstract

An interesting result in particle astrophysics is the recent detection of an unexplained 3.5 keV line from galaxy clusters. A promising model, which can explain the morphology of the signal and its non-observation in dwarf spheroidal galaxies, involves a 7 keV dark matter particle decaying into a pair of ultra-light axions that convert into photons in the magnetic field of the clusters. Given that light axions emerge naturally in 4D string vacua, in this paper we present a microscopic realisation of this model within the framework of type IIB flux compactifications. Dark matter is a local closed string axion which develops a tiny mass due to subdominant poly-instanton corrections to the superpotential and couples via kinetic mixing to an almost massless open string axion living on a D3-brane at a singularity. The interaction of this ultra-light axion with photons is induced by U(1) kinetic mixing. After describing the Calabi-Yau geometry and the brane set-up, we discuss in depth moduli stabilisation, the resulting mass spectrum and the strength of all relevant couplings.