Gamma Rays from Kaluza-Klein Dark Matter

TL;DR

The paper investigates whether Kaluza-Klein dark matter, in particular the $B^{(1)}$ lightest KK particle, can explain the TeV gamma-ray signal from the Galactic Center observed by H.E.S.S. It computes both primary gamma rays from radiative leptonic channels $B^{(1)}B^{(1)} \to \ell^+\ell^-\gamma$ and secondary gamma rays from quark fragmentation and $\tau$ decays, yielding a total spectrum $\frac{dN_\gamma^{\text{eff}}}{dx}$ with $x = E_\gamma / m_{B^{(1)}}$, including a leading-log enhancement and radiative corrections of order $O(\alpha/\pi)$. For $m_{B^{(1)}} \approx 0.8\ \text{TeV}$ and a modest boost factor $b \sim 100$, the predicted flux fits the H.E.S.S. data up to $E_\gamma \lesssim m_{B^{(1)}}$, while fitting the full spectrum would require $m_{B^{(1)}} \gtrsim 7\ \text{TeV}$ or a substantially enhanced gauge coupling to satisfy the relic density (e.g., a 3× larger coupling) or a heavier $m_{B^{(1)}}$ with $b \sim 1000$. The authors discuss observational tests, including potential line signals $B^{(1)}B^{(1)} \rightarrow \gamma\gamma$ and multi-messenger constraints, to distinguish KK DM from other scenarios and to probe DM density profiles near the Galactic Center.

Abstract

A TeV gamma-ray signal from the direction of the Galactic center (GC) has been detected by the H.E.S.S. experiment. Here, we investigate whether Kaluza-Klein (KK) dark matter annihilations near the GC can be the explanation. Including the contributions from internal bremsstrahlung as well as subsequent decays of quarks and tau leptons, we find a very flat gamma-ray spectrum which drops abruptly at the dark matter particle mass. For a KK mass of about 1 TeV, this gives a good fit to the H.E.S.S. data below 1 TeV. A similar model, with gauge coupling roughly three times as large and a particle mass of about 10 TeV, would give both the correct relic density and a photon spectrum that fits the complete range of data.

Figures (3)

• Figure 1: Contributions to $B^{(1)}B^{(1)} \rightarrow \ell^+ \ell^- \gamma$.
• Figure 2: The total number of photons per $B^{(1)}B^{(1)}$ annihilation (solid line), multiplied by $x^2=(E_\gamma/m_{B^{(1)}})^2$. Also shown is what quark fragmentation alone would give (dashed line), and adding to that the contribution from $\tau$ leptons (dotted line). Here we have assumed a $B^{(1)}$ mass of 0.8 TeV and a 5% mass splitting at the first KK level, but the result is quite insensitive to these parameters.
• Figure 3: The H.E.S.S. data hess compared to the gamma-ray flux from a region of $10^{-5}$ sr encompassing the GC, for a $B^{(1)}$ mass of 0.8 TeV, a 5% mass splitting at the first KK level, and a boost factor $b$ around 200 (dashed line). The solid line corresponds to a hypothetical 10 TeV WIMP with similar couplings, a total annihilation rate given by the WMAP relic density bound, and a boost factor around 1000.