Indirect detection of dark matter with γ rays

Abstract

The details of what constitutes the majority of the mass that makes up dark matter in the Universe remains one of the prime puzzles of cosmology and particle physics today-80 y after the first observational indications. Today, it is widely accepted that dark matter exists and that it is very likely composed of elementary particles, which are weakly interacting and massive [weakly interacting massive particles (WIMPs)]. As important as dark matter is in our understanding of cosmology, the detection of these particles has thus far been elusive. Their primary properties such as mass and interaction cross sections are still unknown. Indirect detection searches for the products of WIMP annihilation or decay. This is generally done through observations of γ-ray photons or cosmic rays. Instruments such as the Fermi large-area telescope, high-energy stereoscopic system, major atmospheric gamma-ray imaging Cherenkov, and very energetic radiation imaging telescope array, combined with the future Cherenkov telescope array, will provide important complementarity to other search techniques. Given the expected sensitivities of all search techniques, we are at a stage where the WIMP scenario is facing stringent tests, and it can be expected that WIMPs will be either be detected or the scenario will be so severely constrained that it will have to be rethought. In this sense, we are on the threshold of discovery. In this article, I will give a general overview of the current status and future expectations for indirect searches of dark matter (WIMP) particles.T here is a broad consensus that dark matter (DM) is made up of elementary particles. The most promising candidates are weakly interacting massive particles (WIMPs), particularly if they also form the lightest supersymmetric particle. The general assumption is that the thermal freeze-out in the early Universe leaves a relic density of dark matter particles in the current Universe (after the freeze-out, the particles become too diluted to annihilate in appreciable numbers and thermal energies were too low to produce them; the comoving density is therefore roughly constant since then). The annihilation of these particles into standard model particles controls the abundance in the Universe; thus, there is a tight connection between the annihilation cross section and cosmologically relevant quantities. For particles annihilating (in the simplest case, i.e., annihilating through S waves) (1), the relic density only depends on the annihilation cross section σ ann weighted by the average velocity of the particle (2) Ω χ h 2 = 0:11 3 × 10 −26 cm 3 · s −1 hσ ann vi :As the value for the relic dark matter density from cosmic microwave background (CMB) observations is Ω χ h 2 = 0:113 ± 0:004 (3), it follows that the expected velocity-weighted annihilation cross section is in the range of 3 × 10 −26 cm 3 · s −1 . This represents a striking connection that for typical gauge couplings to ordinary standard model particles and a dark matter mass at the weak interaction scale, WIMPs have the...

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