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5 Direct Searches for New Physics
5.5 Dark Matter and Mono-X
Dark matter has been a very intriguing subject in physics for decades. The history
of dark matter can be traced back to the 1930s, but it was not until the 1970s that
the abundance of non-luminous matter in the universe was recognised as a scientific
problem. It took two more decades until the apparent discrepancy between the visible
matter in galaxies and their gravitational mass inferred from rotation curves had
been accepted to arise from an unknown particle species.
5 Further evidence for
the existence of dark matter is provided by measurements of weak gravitational
lensing [989]. While the nature of dark matter is still unknown and a subject of
theoretical speculations, it is scientifically established that the unaccounted mass
in our universe consists of one or more unknown particles. There are a number
of viable candidates for dark matter. Axions have been postulated to solve the
strong CP problem [990, 991] and have become a popular candidate for dark matter
with masses of 10
−6 –10
−4 eV [992]. While SM neutrinos are not abundant enough
to be viable dark matter particles [993], sterile neutrinos [994] are suggested to
interact only gravitationally, apart from a small mixing with the three generations
of SM neutrinos [995]. The most studied class of dark matter candidates are weakly
interacting massive particles, WIMPs. These arise naturally from theories extending
the SM, such as supersymmetry, which provides several dark matter candidates with
masses from about 50 GeV to a few TeV. An excellent candidate is the neutralino,
which has a self-coupling and interaction strength with ordinary matter just right
that it can account for the observed astrophysical dark matter [996]. WIMPs can also
appear in Little Higgs models [997] and in extra-dimension models, where the dark
matter candidate is a stable Kaluza-Klein particle [998].
Several possibilities exist to search for traces of dark matter particles. The first
method relies on the annihilation of dark matter particles, shown schematically in
Fig. 5.20a. Because the annihilation rate of dark matter particles in the early universe
is needed to be sufficiently large to ensure a stable equilibrium, the annihilation of
relic dark matter particles to SM particles can lead to detectable signals from astrophysical sources. The signals include high-energy photons, neutrinos, positrons and
anti-protons. Dedicated experiments have been designed for these hints of dark matter annihilation. Examples are the telescopes HESS [1000], MAGIC [1001] and
VERITAS [1002] searching for high-energy cosmic rays, AMANDA [1003], SuperKamiokande [1004] and IceCube [1005, 1006] have been designed to detect neutrinos, and the space-based experiments AMS [1007], PAMELA [1008, 1009], AMS02 [1010] study the cosmic positron and anti-proton spectra. The second method are
direct detection experiments, designed to detect the scattering of dark matter and SM
particles, Fig. 5.20b. These experiments rely on the existence of relic dark matter
in our solar system, with a density large enough to result in detectable signals. The
signature is a nuclear recoil from the scattering, where different target materials and
5 The interested reader is referred to [987, 988] for an exhaustive historical perspective on the scientific struggle, including the observational discoveries and the theoretical arguments, that preceded
dark matter becoming part of the standard cosmological model.
5 Direct Searches for New Physics
5.5 Dark Matter and Mono-X
Dark matter has been a very intriguing subject in physics for decades. The history
of dark matter can be traced back to the 1930s, but it was not until the 1970s that
the abundance of non-luminous matter in the universe was recognised as a scientific
problem. It took two more decades until the apparent discrepancy between the visible
matter in galaxies and their gravitational mass inferred from rotation curves had
been accepted to arise from an unknown particle species.
5 Further evidence for
the existence of dark matter is provided by measurements of weak gravitational
lensing [989]. While the nature of dark matter is still unknown and a subject of
theoretical speculations, it is scientifically established that the unaccounted mass
in our universe consists of one or more unknown particles. There are a number
of viable candidates for dark matter. Axions have been postulated to solve the
strong CP problem [990, 991] and have become a popular candidate for dark matter
with masses of 10
−6 –10
−4 eV [992]. While SM neutrinos are not abundant enough
to be viable dark matter particles [993], sterile neutrinos [994] are suggested to
interact only gravitationally, apart from a small mixing with the three generations
of SM neutrinos [995]. The most studied class of dark matter candidates are weakly
interacting massive particles, WIMPs. These arise naturally from theories extending
the SM, such as supersymmetry, which provides several dark matter candidates with
masses from about 50 GeV to a few TeV. An excellent candidate is the neutralino,
which has a self-coupling and interaction strength with ordinary matter just right
that it can account for the observed astrophysical dark matter [996]. WIMPs can also
appear in Little Higgs models [997] and in extra-dimension models, where the dark
matter candidate is a stable Kaluza-Klein particle [998].
Several possibilities exist to search for traces of dark matter particles. The first
method relies on the annihilation of dark matter particles, shown schematically in
Fig. 5.20a. Because the annihilation rate of dark matter particles in the early universe
is needed to be sufficiently large to ensure a stable equilibrium, the annihilation of
relic dark matter particles to SM particles can lead to detectable signals from astrophysical sources. The signals include high-energy photons, neutrinos, positrons and
anti-protons. Dedicated experiments have been designed for these hints of dark matter annihilation. Examples are the telescopes HESS [1000], MAGIC [1001] and
VERITAS [1002] searching for high-energy cosmic rays, AMANDA [1003], SuperKamiokande [1004] and IceCube [1005, 1006] have been designed to detect neutrinos, and the space-based experiments AMS [1007], PAMELA [1008, 1009], AMS02 [1010] study the cosmic positron and anti-proton spectra. The second method are
direct detection experiments, designed to detect the scattering of dark matter and SM
particles, Fig. 5.20b. These experiments rely on the existence of relic dark matter
in our solar system, with a density large enough to result in detectable signals. The
signature is a nuclear recoil from the scattering, where different target materials and
5 The interested reader is referred to [987, 988] for an exhaustive historical perspective on the scientific struggle, including the observational discoveries and the theoretical arguments, that preceded
dark matter becoming part of the standard cosmological model.