Two-photon annihilation of Kaluza–Klein dark matter

Abstract

We investigate the fermionic one-loop cross section for the two-photon annihilation ofKaluza–Klein (KK) dark matter particles in a model of universal extra dimensions(UED). This process gives a nearly mono-energetic gamma-ray line with energyequal to the KK dark matter particle mass. We find that the cross section islarge enough that if a continuum signature is detected, the energy distribution ofgamma-rays should end at the particle mass with a peak that is visible for an energyresolution of the detector at the per cent level. This would give an unmistakablesignature of a dark matter origin of the gamma-rays, and a unique determinationof the dark matter particle mass, which in the case studied should be around800 GeV. Unlike the situation for supersymmetric models where the two-gammapeak may or may not be visible depending on parameters, this feature seems tobe quite robust in UED models, and should be similar in other models whereannihilation into fermions is not helicity suppressed. The observability of thesignal still depends on largely unknown astrophysical parameters related to thestructure of the dark matter halo. If the dark matter near the galactic centre isadiabatically contracted by the central star cluster, or if the dark matter halohas substructure surviving tidal effects, prospects for detection look promising.