Prospects for indirect MeV dark matter detection with gamma rays in light of cosmic microwave background constraints

The self-annihilation of dark matter particles with mass in the MeV range can produce gamma rays via prompt or secondary radiation. The annihilation rate for such light dark matter particles is however tightly constrained by cosmic microwave background (CMB) data. Here we explore the possibility of discovering MeV dark matter annihilation with future MeV gamma-ray telescopes taking into account the latest and future CMB constraints. We study the optimal energy window as a function of the dominant annihilation final state. We consider both the (conservative) case of the dwarf spheroidal galaxy Draco and the (more optimistic) case of the Galactic center. We find that for certain channels, including those with one or two monochromatic photon(s) and one or two neutral pion(s), a detectable gamma-ray signal is possible for both targets under consideration and compatible with CMB constraints. For other annihilation channels, however, including all leptonic annihilation channels and two charged pions, CMB data rule out any significant signal of dark matter annihilation at future MeV gamma-ray telescopes from dwarf galaxies, but possibly not for the Galactic center.

Figure 1

Energy-injecting efficiency functions $f_{\mathrm{eff}}$ for the six different dark matter annihilation channels considered in this work.

Figure 2

Results of the analysis on the integration range in the photon spectrum and diffuse-background flux. We present the number of event photons (left axis, solid lines) in the energy range $\mathrm{\Delta }E/m_{\chi }$. On the right axis (dashed lines) we show the corresponding signal-to-noise ratio $N_s/\sqrt{N_b}$ ($\#\sigma$) vs $\mathrm{\Delta }E/m_{\chi }$. The vertical dashed lines represent the maximum energy-range possible for a certain mass in the case of neutral pions.

Figure 3

Comparison on the values of $⟨\sigma v⟩$ needed for a $5\sigma$ detection in the hypothetical gamma-ray detector described in the text from dark matter annihilation in the dSph Draco and the current constraints from Planck. The dashed lines represent the $⟨\sigma v⟩$ needed for a $5\sigma$ detection while the solid lines represent the Planck constraints. The gray line is the projection constraint from COrE+, TEP. The yellow-colored region is ruled out by the Planck constraints.

Figure 4

As in Fig. 3, but for the $e^{+}{e}^{-}$, $\mu +{\mu }^{-}$ and ${\pi }^{+}{\pi }^{-}$ final states.

Figure 5

Comparison on the values of $⟨\sigma v⟩$ needed for a $5\sigma$ detection in the hypothetical gamma-ray detector described in the text, from dark matter annihilation in Galactic center, for the $\gamma \gamma$, $\gamma {\pi }^0$ and ${\pi }^0{\pi }^0$ final states. The dashed lines correspond to the different density profiles used to compute the “J-factor”, the label indicates the name of the simulation and the inner slope of the density profile (see the text for more details). The blue region is ruled out by the PLANCK constraints (blue line), while the gray line is the projection constraint from $\mathrm{COrE}+$, TEP.

Figure 6

Same as in Fig. 5 but for the $e^{+}{e}^{-}$, $\mu +{\mu }^{-}$ and ${\pi }^{+}{\pi }^{-}$ final states.