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article imageDark matter — Stalked from every which way

By Karen Hardison     Apr 27, 2015 in Science
Dark matter is hunted in space, in the Solar system and in distant galaxies. Signs of its interactions, its fingerprint and its decay are pursued. Whether WIMP, axion or neutrino, though unseen, the net is tightening around it.
Dark matter is pursued by astronomers and particle physicists. It's hunted in the Milky Way, the Sun and dwarf galaxies. It's hunted from the Space Station, from ground-based giant telescopes, by CERN and by Hubble.
WIMPS — Weakly Interacting Massive Particles
WIMPs are the favored particle that physicists search for. Failed efforts to trap WIMPs in underground detectors turned scientists to hunting in the skies. In the nearby sky, the search for dark matter hangs onto the side of the Space Station and looks for cosmic ray decay.
If dark matter is made up of WIMPs, or weakly interacting massive particles (particles that have mass but interact weakly with visible matter), then dark matter may be its own anti-matter. This means that when enough dark matter gets together in one place, it self-annihilates. This annihilation of WIMPS might produce cosmic rays, gamma rays, or neutrinos. Manuel Gnida of Symmetry Magazine explains the indirect detection experiments being used to cast a net around the possible particles of dark matter.
Cosmic Rays
The self-annihilating potential of WIMPs is why the Alpha Magnetic Spectrometer (AMS) is hitchhiking on the outside of the Space Station. It is collecting cosmic ray positron data to identify unexplained cosmic ray concentrations because such concentrations would point to dark matter that has self-annihilated.
Galactic Interactions
On the other hand, if the density and dispersion of WIMPs is sparse enough, dark matter might not self-annihilate. It may interact with itself through a dark force that governs dark matter in the way that the electromagnetic force, the strong and weak forces and the gravitational force govern interactions between objects made up of regular matter.
This is why astronomers have turned to studying collisions between clusters of galaxies and between individual galaxies. They predict that if WIMP dark matter interacts with itself, then in galaxy collisions — where both dark and visible matter are more dense and less thinly dispersed — there will be a detectable offset of clumps of dark matter from the colliding galaxies signifying that dark matter has been slowed down and lags behind due to an interaction with the colliding dark matter from the other galaxy.
In contrast, they predict that the offsetting, lagging behind effect will not be apparent in collisions between clusters of galaxies because, in clusters, matter is less dense and dispersion is greater, which is the reason large clusters of galaxies paradoxically finish collisions sooner than individual galaxies do (large clusters are less dense with greater dispersion, individual galaxies are more dense with lesser dispersion).
Gamma Rays
The center of the Milky Way is where astronomers look to find gamma rays, which would signify that dark matter at the heart of the galaxy has annihilated and produced an otherwise unexplained concentration of gamma rays. There are many astrophysical sources that produce gamma rays, so this task requires eliminating all other known sources of gamma ray at the galactic center before the confirmation of dark matter can be made.
Dwarf satellite galaxies — young, new galaxies that circle the Milky Way — have no known gamma ray sources and have dense concentration of dark matter. If gamma ray concentrations are detected in dwarf satellites, then dark matter annihilation would be the probable source. Fermi-LAT (Large Area Telescope) researcher Matthew Wood, co-leader of two recent analyses of known and new dwarf galaxies found by the Dark Energy Survey explained the connection between gamma ray and dwarf galaxies to Symmetry Magazine: “Dwarf galaxies are dominated by dark matter and don’t contain any known gamma-ray sources [with] more than a million times fewer stars than our own galaxy, [they] are ideal targets for indirect dark matter searches.”
Massive ground-based gamma ray detectors, VERITAS, MAGIC, H.E.S.S. and soon the Cherenkov Telescope Array, collect data to pinpoint unexplained concentrations of gamma rays. If WIMPs self-annihilate into gamma rays, this three-pronged search narrows the odds against dark matter continuing its elusive evasion.
Neutrinos streaming from the core of the Sun could only be caused by self-annihilation of dark matter that became trapped in the Sun's core, intensified in density, and annihilated. Whereas gamma and cosmic rays might have many sources, neutrinos from the Sun can only be explained by dark matter self-annihilation. If those intriguing flavor-oscillating almost massless neutrinos are caught flying from the Sun, then we caught dark matter in action trapped within the core of the Sun.
As the leader of the IceCube neutrino detector — searching at the South Pole — Francis Halzen of the University of Wisconsin, Madison, said to Symmetry Magazine: “Only neutrinos are able to escape from the center of the sun. If we ever find such a high-energy neutrino signal, there will be no debate as to whether we have found a dark matter signature or not.”
Cosmic Microwave Background
If dark matter is made up of WIMPs, and if WIMPs existed in the early universe, then the fingerprint of dark matter WIMPs should be detectable in the cosmic microwave background (CMB). The Planck space telescope has been invaluable in making a whole sky map — aided now by the latest Dark Energy Survey map — and part of its quest is to find the fingerprint of dark matter in the CMB. This is where CERN comes in. The world's largest, fastest particle accelerator, CERN has dedicated itself, in its 2015 run of Large Hadron Collider (LHC) particle collisions, to hunting for dark matter by recreating the high-energy, beginning at 13 TeV, conditions of the early universe.
Dark Matter: Elusive but Hunted
Dark matter may be invisible and elusive, but it will soon have no place to run to and no place to hide. Its interactive properties, its particle decay signature, its early-universe fingerprint, and its role in the Sun's core are being systematically tracked. Whether axions, WIMPs, or strange normal matter, dark matter's puzzle will be put together and the mystery of dark matter will be unveiled — sooner than later.
More about Dark matter, particle decay, Cern, AMS, veritas
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