Two photon annihilation of Kaluza-Klein dark matter

TL;DR

The paper calculates the fermionic one-loop cross section for the two-photon annihilation of KK dark matter in a universal extra dimensions model, predicting a mono-energetic gamma-ray line at $E_\gamma \approx m_{B^{(1)}}$. The dominant contribution comes from first KK-level fermion box diagrams, yielding a cross section that scales as $m_{B^{(1)}}^{-2}$ and is relatively insensitive to higher KK states. When combined with the continuum gamma spectrum, the line feature provides a sharp signature that, with percent-level detector energy resolution and favorable dark matter halo profiles, could be detectable and would directly determine the LKP mass (around $0.8\ \mathrm{TeV}$ in their benchmark). The study also notes a modest enhancement from the $Z\gamma$ channel and emphasizes halo uncertainties as the main obstacle to observability, with prospects improving if adiabatic contraction or substructure exist.

Abstract

We investigate the fermionic one-loop cross section for the two photon annihilation of Kaluza-Klein (KK) dark matter particles in a model of universal extra dimensions (UED). This process gives a nearly mono-energetic gamma-ray line with energy equal to the KK dark matter particle mass. We find that the cross section is large enough that if a continuum signature is detected, the energy distribution of gamma-rays should end at the particle mass with a peak that is visible for an energy resolution of the detector at the percent level. This would give an unmistakable signature of a dark matter origin of the gamma-rays, and a unique determination of the dark matter particle mass, which in the case studied should be around 800 GeV. Unlike the situation for supersymmetric models where the two-gamma peak may or may not be visible depending on parameters, this feature seems to be quite robust in UED models, and should be similar in other models where annihilation into fermions is not helicity suppressed. The observability of the signal still depends on largely unknown astrophysical parameters related to the structure of the dark matter halo. If the dark matter near the galactic center is adiabatically contracted by the central star cluster, or if the dark matter halo has substructure surviving tidal effects, prospects for detection look promising.

Figures (3)

• Figure 1: Fermion box contributions to $B^{(1)}B^{(1)} \rightarrow \gamma\gamma$ including the first level of KK excitations. Not shown are the additional nine diagrams that are obtained by crossing external momenta.
• Figure 2: The annihilation rate for $m_{B^{(1)}}=$ 0.8 TeV as a function of the mass shift between the $B^{(1)}$ and KK fermions. The dependence on the $B^{(1)}$ mass is simply given by $\sigma v\propto m_{B^{(1)}}^{-2}$.
• Figure 3: The continuous gamma-ray flux that is expected from KK dark matter bbeg is plotted as a solid line. To this, we have added the flux from direct annihilation as seen by a detector with an energy resolution of $2\sigma$ = 2 % (dashed), 1 % (dotted) and 0.5 % (dash-dotted), respectively. The actual linewidth of the signal is about $10^{-3}$, with a peak value of $1.5\cdot10^{-7}~\mathrm{m}^{-2}~\mathrm{s}^{-1}~\mathrm{TeV}^{-1}$. For all cases, we have assumed $m_{B^{(1)}}=800$ GeV, a mass shift $m_{\xi^{(1)}}/m_{B^{(1)}}=1.05$, and a moderate boost factor of $b=100$.