Dark matter annihilation and decay

The fact that the matter content of the universe is dominated by a dark matter (DM) component is now well established, and observed from galactic to cosmological scales. DM is also the main ingredient of our current structure formation theory. Although departures from General Relativity might still partly explain current observations, the most appealing solution to the DM issue is the existence of new exotic and very long-lived or stable particles, which generically arise in extensions of  the standard model (SM) of particle physics (PP). These may be related to specific and independent PP problems (e.g. neutrino mass, strong CP problem, unification of interactions, hierarchy problem(s), etc.), or elaborated on purpose (effective models, minimal approaches). The big advantage of the DM particle scenario is that it can be tested with currently operating or future instruments. For example, annihilation or decay products of DM in galaxies could be observed, which would help unveil the DM particle nature – while powerful constraints may come out if not, which could even exclude quite generic ideas.

The mass range and specific properties allowed for these particles strongly depend on models and other constraints (e.g. the cosmological abundance), and, except for axion-like-particles, the relevant energy range for detection with astrophysical photons roughly goes from 10 keV (sterile neutrinos as warm/cold DM) to 1 TeV (supersymmetric candidates). Some candidates may annihilate or decay into photon lines (gamma+neutrino is typical for decays), which provides a clear signature.  The MeV energy range is particularly interesting for those DM candidates that have been invoked to contribute the intense 511 keV line emission arising from the Galactic Center, whose full origin remains undetermined. Since the dominant part of this emission must come from positronium decay, electrons and positrons originating in DM annihilation or decay must be injected with MeV energies at most, constraining the DM mass range or the mass splitting in case of excited DM models (no strong mass constraint in the latter case). Annihilation into charged lepton pairs is prototypal for MeV astronomy since it is necessarily accompanied by final state radiation corrections, which induce hard photon spectra, and therefore potentially observable signals whose intensity should somehow trace the DM distribution in targets.

Beside spectral anomalies in the diffuse Galactic gamma-ray sky or toward the Galactic Center, another promising class of targets would be that of Dwarf Spheroidal (DSph) Galaxies orbiting the Milky Way, which are background-free DM dominated objects – the detection of a single such object would then have a tremendous impact. In this topic, MeV astronomical observations would still be strongly complementary to direct DM detection experiments, which are now able to probe DM couplings to electrons down to the MeV mass range, and to colliders like the LHC.

Required instrument performances:
For DM candidates decaying into gamma-ray lines (small part of available models): very good energy resolution (% scale) may compensate for a low angular resolution. For other candidates, angular resolution becomes important to maximize the signal-to-noise ratio. Since sources are identified (GC, DSphs), the field of view is not a strong limit, provided significant observation time is available.

Performance parameter   Goal value Remarks and notes
(FWHM, deg)
Angular resolution
(FWHM, deg)
0.1 1 0.1° corresponds to the angular size roughly encompassing nearby DSPhs
Spectral resolution
(ΔE/E @ Energy)
10-6 Energy dispersion induced by the velocity dispersion of DM particles in targets. Valid for DM candidates decaying into photon lines (e.g. sterile neutrinos)
Line sensitivity (@ Energy)
(cm-2 s-1, 3σ, 1 Ms)
10-6 @ 511 keV To detect the e+e- line emission from DSph. Comparable sensitivity required for direct decay into lines in this energy range
Continuum sensitivity (in which energy band?)
(cm-2 s-1 keV-1, ΔE=E, 3σ, 1 Ms)
 10-7 – 10-6
around 1 MeV
Sensitivity to detect continuous emission from DSphs
Timing performances    
Polarimetric capability
(Minimum Polarization Fraction for a Crab source in 1 Ms)
Real-time data?