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.
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| 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
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| 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). |