Probing super-heavy dark matter with ultra-high-energy gamma rays

TL;DR

This work assesses indirect limits on decaying super-heavy dark matter with masses between $10^{7}$ and $10^{15}$ GeV by exploiting prompt Galactic $\gamma$-rays and the latest UHE $\gamma$-ray upper limits from multiple extensive air shower experiments. It introduces a careful treatment of detector geometry through the geometrical acceptance $\omega(\delta)$, and computes energy- and position-dependent $\mathcal{D}$-factors for two representative DM halo profiles (gNFW and Burkert), including attenuation by the CMB and Galactic photon fields. The analysis yields 95% CL lower bounds on $\tau_{\rm DM}$ for several decay channels, showing that accounting for FOV reduces the bounds relative to previous studies but provides a more faithful reflection of current experimental sensitivities; the results remain complementary to neutrino-based probes and suggest that parts of the SHDM parameter space could still yield observable neutrino fluxes. The study also clarifies the interplay between gamma-ray constraints and potential SHDM interpretations of high-energy neutrino events, such as KM3NeT’s 230 PeV event, emphasizing the role of robust DM density modeling and detector acceptance in future SHDM tests.

Abstract

We refine the constraints on the lifetime of decaying super-heavy dark matter particles (SHDM), with masses ranging from $10^7$ to $10^{15}$ GeV, by analyzing ultra-high-energy (UHE) gamma-ray data. Our approach involves an accurate comparison of the primary gamma-ray emissions resulting from prompt SHDM decays in the galactic halo with the most recent upper limits on isotropic UHE gamma-ray fluxes provided by various extensive air shower experiments. We demonstrate that a precise consideration of the field of view and the geometric acceptance of different UHE gamma-ray observatories has significant implications for the inferred limits of dark matter lifetime. In addition, we examine the influence of uncertainties linked to the current models of the galactic dark matter distribution, employing diverse halo density profiles while varying both their radial extent and the local dark matter density. Our findings indicate that the newly established UHE gamma-ray constraints are marginally less stringent than earlier evaluations, thereby revisiting the SHDM parameter space and allowing for observable neutrino fluxes.

Figures (9)

• Figure 1: Geometrical interpretation of the visibility condition for a detector located at latitude $\lambda = 40^{\circ}$ and with a zenith cut $\theta_m = 60^{\circ}$. The left plot shows the altitude curves and the corresponding visible hour-angle intervals for selected declinations. The visibility interval $\Delta H = 2\alpha_m$ corresponds to the portion of trajectory lying above the minimum altitude $h_m$. The graphic on the right shows the same trajectories projected on the celestial sphere together with the detector FOV (green shaded area). The red arc denotes the portion of each trajectory lying above the minimum altitude and thus contributing to the exposure.
• Figure 2: Geometrical acceptance efficiency $\omega(\delta)$ as a function of the declination $\delta$, for different UHE gamma-ray detectors shown with distinct curve styles. The configurations HECO + SD 750 m and Hybrid of the PAO detector share the same geometrical acceptance efficiency as their zenith angle ranges are equal (see Tab. \ref{['tab:experiments_parameters']}).
• Figure 3: FOV in galactic coordinates for each experiment considered in this study, with color grading indicating the geometrical acceptance efficiency $\omega (\delta(b,l))$ defined in Eq. \ref{['eq:omega_delta']}.
• Figure 4: Left panel: transmission factor for photons traversing both the CMB and the galactic SL+IR photon field computed for different distances of sources $s$, longitudes and latitudes $(b,l)$. Right panel: transmission factor for photons of energy $E_{\gamma}$ considering only the CMB field.
• Figure 5: $\mathcal{D}$-factor as a function of the gamma-ray energy computed for the full sky and for the experiments listed in Tab. \ref{['tab:experiments_parameters']}, shown with different colors. The x-axis and y-axis are swapped for better visualization. The left and right plots refer to the gNFW and Burkert DM profiles, respectively. The curves with distinct styles correspond to the fiducial DM profiles, while the shaded regions quantify the $2\sigma$ uncertainty on the DM-halo parameters from Ref. Benito:2020lgu (see text and Tab. \ref{['tab:D_factor']}).