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Indirect detection
Indirect
detection experiments search for the products of the self-annihilation
or decay of dark matter particles in outer space. For example,
in regions
of high dark matter density (e.g., the centre of our galaxy) two
dark matter
particles could annihilate to produce gamma rays or Standard Model
particle–
antiparticle pairs. Alternatively, if a dark matter particle is
unstable,
it could decay into Standard Model (or other) particles. These
processes
could be detected indirectly through an excess of gamma rays,
antiprotons
or positrons emanating from high density regions in our galaxy
or others.
A major difficulty inherent in such searches is that various astrophysical
sources can mimic the signal expected from dark matter, and so
multiple
signals are likely required for a conclusive discovery.
A few of the dark matter particles passing through the Sun or
Earth may
scatter off atoms and lose energy. Thus dark matter may accumulate
at the
center of these bodies, increasing the chance of collision/annihilation.
This
could produce a distinctive signal in the form of high-energy
neutrinos.
Such a signal would be strong indirect proof of WIMP dark matter.
Highenergy
neutrino telescopes such as AMANDA, IceCube and ANTARES
are searching for this signal. The detection by LIGO in September
2015
of gravitational waves, opens the possibility of observing dark
matter in a
new way, particularly if it is in the form of primordial black
holes.
Many experimental searches have been undertaken to look for such
emission
from dark matter annihilation or decay, examples of which follow.
The
Energetic Gamma Ray Experiment Telescope observed more gamma rays
in 2008 than expected from the Milky Way, but scientists concluded
this
was most likely due to incorrect estimation of the telescope’s
sensitivity.
The Fermi Gamma-ray Space Telescope is searching for similar gamma
rays. In April 2012, an analysis of previously available data
from its Large
Area Telescope instrument produced statistical evidence of a 130
GeV
signal in the gamma radiation coming from the center of the Milky
Way.
WIMP annihilation was seen as the most probable explanation.
At higher energies, ground-based gamma-ray telescopes have set
limits on
the annihilation of dark matter in dwarf spheroidal galaxies and
in clusters
of galaxies.
The PAMELA
experiment (launched in 2006) detected excess positrons.
They could be from dark matter annihilation or from pulsars. No
excess
antiprotons were observed.
In 2013
results from the Alpha Magnetic Spectrometer on the International
Space Station indicated excess high-energy cosmic rays which could
be
due to dark matter annihilation.
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Collider searches for dark matter
An alternative
approach to the detection of dark matter particles in nature
is to produce them in a laboratory. Experiments with the Large
Hadron
Collider (LHC) may be able to detect dark matter particles produced
in
collisions of the LHC proton beams. Because a dark matter particle
should
have negligible interactions with normal visible matter, it may
be detected
indirectly as (large amounts of) missing energy and momentum that
escape
the detectors, provided other (non-negligible) collision products
are detected.
Constraints on dark matter also exist from the LEP experiment
using a
similar principle, but probing the interaction of dark matter
particles with
electrons rather than quarks. Any discovery from collider searches
must
be corroborated by discoveries in the indirect or direct detection
sectors to
prove that the particle discovered is, in fact, dark matter.
