Dark Matter

Gamma Rays from Dark Matter

Around 30% of the total mass of the Universe is thought to consist of dark matter, most likely in the form of a new class of particles such as predicted by the supersymmetric or extra dimensional extensions to the standard model of particle physics.

Depending on the model, dark matter can annihilate or decay to detectable standard model particles, in particular gamma-rays. Large dark matter densities due to the particles’ accumulation in potential wells, such as near the centres of galaxies, lead to detectable fluxes, especially given that the annihilation rate (and therefore the gamma-ray flux) is proportional to the square of the density.

CTA would be a dark matter discovery instrument of unprecedented sensitivity and provde a tool to study the particle and astrophysics properties of the dark matter particles. If candidate dark matter particles are discovered at the Large Hadron Collider or in underground experiments, CTA will aim to verify whether they actually form the dark matter in the Universe.

Spectral Signatures of Dark Matter

Slow-moving dark matter particles could give rise to a striking, almost mono-energetic photon emission. The discovery of such line emission would be conclusive evidence for dark matter, and CTA might have the capability to detect gamma-ray lines even if the cross-section is loop-suppressed, which is the case for the most popular candidates of dark matter, i.e. those inspired by the minimal supersymmetric extensions to the standard model (MSSM) and models with extra dimensions, like Kaluza-Klein (KK) theory. Line radiation from these candidates is not detectable by Fermi, H.E.S.S. II or MAGIC II, unless optimistic assumptions about the dark matter density distribution are made. Recent calculations regarding the gamma-ray spectrum from the annihilation of MSSM dark matter indicate the possibility of final-state contributions also giving rise to distinctive spectral features. The more generic continuum contribution (arising from pion production) is more ambiguous, but with its curved shape it is potentially distinguishable from the usual power-law spectra exhibited by known astrophysical sources.

The Galactic Centre

Our Galactic Centre (GC) is one of the most promising regions to look for dark matter annihilation radiation due to its predicted very high dark matter density. High energy gamma-ray emission has been detected by many experiments (e.g. H.E.S.S., MAGIC and VERITAS). However, the identification of dark matter in the GC is complicated by the presence of many conventional source candidates and the diffuse gamma-ray background. The angular and energy resolution of CTA, together with its enhanced sensitivity, will be crucial for disentangling the different contributions to the GC radiation.

A target for dark matter searches: the Sculptor dwarf spheroidal galaxy. (Image: David Malin, Anglo-Australian Observatory)

 

Dwarf Galaxies

Other individual targets for dark matter searches are dwarf galaxies, including the dwarf spheriodal systems. These exhibit large mass-to-light ratios, and make dark matter searches with low astrophysical backgrounds possible. Some of these objects have been observed with H.E.S.S., MAGIC and Fermi, and upper limits on dark matter annihilation have been calculated, which are about an order of magnitude higher than the emission expected in most relevant cosmological models. Thanks to its high sensitivity in the low- and medium-energy ranges, CTA will be able to improve significantly on these measurements.

Diffuse Emission

Dark matter is also expected to contribute to the extra-galactic and galactic diffuse emission, displaying both spectral and spatial signatures. While the emissivity of conventional astrophysical sources scale with the local matter density, the emissivity of annihilating dark matter scales with the density squared, resulting in differences in the small-scale anisotropy power spectrum of the diffuse emission.

Recent measurements of the positron fraction presented by the PAMELA Collaboration point towards a relatively local source of positrons and electrons, a result which is reinforced by the Fermi measurement of the eletron-positron spectrum. It is suggested that either pulsar(s) or dark matter annihilation could be responsible for tis emission. One way to distinguish between these two hypotheses is to use the spectral shape. The dark matter spectrum exhibits a sudden drop at an energy which corresponds to the dark matter particle mass, whereas the pulsar spectrum falls off more smoothly. Another hint would be a small anisotropy, either in the direction of the GC (for dark matter) or in the direction of the nearest mature pulsars. The large effective area of CTA, about 6 orders of magnitudes larger than balloon- and satellite-borne experiments, might allow the measurement of the spectral shape and even the tiny anisotropy.

If the PAMELA result originates from dark matter, the particles' mass would be > 1 TeV, which is large in comparison with most dark matter candidates. Having its greatest sensitivity at 1 TeV, CTA would be better suited for the detection of dark matter particles of such masses than Fermi, which has its best sensitivity at masses of order 10 to 100 GeV.

 

Further Reading

 

Bergstrom, Dark Matter Candidates, New Journal of Physics (2009), 11, 10, p. 105006;  http://arxiv.org/abs/0903.4849

Alvarez et al., Science with the New Generation High Energy Gamma-Ray Experiments;  http://arxiv.org/abs/0712.1548

Adriani et al., Observation of an Anomalous Positron Abundance in the Cosmic Radiation,
Nature (2009), 458, 7238, p. 607-609; http://arxiv.org/abs/0810.4995

Bergstrom et al., Complementarity of direct dark matter detection and indirect detection through gamma-rays; http://arxiv.org/abs/1011.4514

One ring to rule them all: This HST image shows the CL0024+17 galaxy cluster. Several unusual and repeated galaxy shapes can be observed, showing that the cluster is a strong gravitational lens. The relatively weak distortions of the many distant, faint, blue galaxies all over the image indicates the presence of a dark matter ring. Image credit: NASA/ESO/M.J. Lee and H. Ford et al.(Johns Hopkins University).