Indirect detection of dark matter using MeV-range gamma-ray telescopes

The astrophysics community is considering plans for a variety of gamma-ray telescopes (including ACT, GRIPS, and AdEPT) in the energy range 1–100 MeV, which can fill in the so-called “MeV gap” in current sensitivity. We investigate the utility of such detectors for the study of low-mass dark matter annihilation or decay. For annihilating (decaying) dark matter with a mass below about 140 MeV (280 MeV) and couplings to first generation quarks, the final states will be dominated by photons or neutral pions, producing striking signals in gamma-ray telescopes. We determine the sensitivity of future detectors to the kinematically allowed final states. In particular, we find that planned detectors can improve on current sensitivity to this class of models by up to a few orders of magnitude.

Figure 1

CMB and diffuse constraints on dark matter annihilation (left) and decay (right). The top and bottom sets of panels display the same information, plotted on a log and linear $x$ axis, respectively. The strongest CMB bound comes from Planck, shown in black for annihilation [46] and decay [41]. Conservative bounds from COMPTEL, EGRET, and Fermi for diffuse emission are shown in blue, red, and green, respectively, for the channels $\gamma \gamma$ (solid), $\gamma {\pi }^0$ (dashed), and ${\pi }^0{\pi }^0$ (dotted). Optimistic bounds, described in Sec. 5b, can be obtained by improving the COMPTEL, EGRET, and Fermi bounds by a factor of $\sim 3–5$, 9, and 6, respectively. Vertical dotted lines show the kinematic thresholds at $m_{{\pi }^0}$ and $2m_{{\pi }^0}$, and $\sqrt{s}$ is plotted up to $m_{{\pi }^{\pm }}$.

Figure 2

Projected sensitivities on dark matter annihilation (left) and decay (right) for channels $\gamma \gamma$ (top), $\gamma {\pi }^0$ (middle), and ${\pi }^0{\pi }^0$ (bottom). Planck (black) bounds are shown from Fig. 1. The cyan, blue, red, and green shaded regions show the areas between the conservative and optimistic sensitivities for the diffuse background for ACT, GRIPS, AdEPT, and GAMMA-400, respectively. The hatched regions show the $1\sigma$ uncertainty bounds for Draco after five years of observation. Vertical dotted lines show the kinematic thresholds at $m_{{\pi }^0}$ and $2m_{{\pi }^0}$ (plotting begins at $m_{{\pi }^0}$ for the $\gamma {\pi }^0$ channel and at $2m_{{\pi }^0}$ for the ${\pi }^0{\pi }^0$ channel), and $\sqrt{s}$ is plotted up to $m_{{\pi }^{\pm }}$ for all three channels. Note that the vertical axes cover a different range for $\gamma \gamma$ than for the other two channels.