CTAO Science
Dark matter
Most of the matter in the Milky Way is dark, likely composed of yet unknown particles predicted in extensions of the Standard Model of particle physics. Interactions of the dark matter particles in the Milky Way and in other galaxies result in signals observable by telescopes. On the way to us in the intergalactic space axions might interact with gamma-rays distorting the observed AGN spectra. CTAO will shed light on the quest on the dark matter nature. Is dark matter composed of Weakly Interacting Massive Particles (WIMPs) or in the form of Axion-Like Particles (ALPs)? What is the lifetime of dark matter particles?
All the known matter in the Milky Way resides in a small region at the bottom of a gravitational potential well produced by dark matter. Most of the galaxy's mass and volume is filled with the elusive dark matter that manifests itself only through its gravitational pull. Many theoretical models assume that the dark matter particles still interact among themselves and with the conventional matter but with very tiny cross-sections. Large direct detection experiments, like XENON and its future successor DARWIN, are being deployed in attempts to catch some of the dark matter particles and make them produce signals in detectors placed on the Earth. This is possible if the dark matter is made of WIMPs with masses between several GeV to TeV energies. Alternatively, the sparks of radiation or particles and neutrinos from pair-wise collisions of dark matter particles leading to their destruction (annihilation) everywhere in the Galaxy can be seen by telescopes based on Earth or on space-based detectors. The AMS experiment on the International Space Station, with Swiss scientists at the forefront, hints to an excess of anti-electrons and seeks for a background-free signature of heavy anti-matter from dark-matter annihilation. In the case of WIMPs, the cross-section of the annihilation is known, so that the strength of the “annihilation glow” of the Milky Way can be predicted with high confidence. CTAO will have sufficient sensitivity to detect the WIMP annihilation in the central part of the Milky Way, if WIMPs indeed constitute the bulk of the dark matter. Otherwise, it will strongly constrain the possible existence of WIMPs. Gamma-ray observations also can reveal another type of signal produced by decaying dark matter particles that are very light, with masses that are comparable or smaller than the masses of the lightest known particles, neutrinos. In this case, the decaying dark matter particles may emit visible-infrared photons that would contribute to the EBL. Such contribution is detectable with CTAO. Heavy unstable dark matter particles would decay with the production of gamma-rays, also detectable with CTAO.
Another candidate for dark matter is the hypothetical Axion-Like Particle (ALP). In a strong magnetic field, a TeV photon could convert into an ALP, and convert back into a photon in another strong eld. These ALP- gamma-ray oscillations would introduce features in the spectra of distant AGNs which could lead to indications for the existence of ALPs.