The University of Texas at Austin
  • Seeing dark matter in the ice

    By Daniel Oppenheimer
    Daniel Oppenheimer
    Published: Dec. 1, 2010

    In the dense centers of the dwarf spheroidal galaxies that live in the dark matter halo of the Milky Way there are, perhaps, particles known as weakly interacting massive particles (WIMPs) that are colliding with each other and “annihilating” into other kinds of particles, which shoot out into the void.

    Illustration showing the IceCube ray
    The IceCube array shown in relation to the drill camp and the bedrock beneath. View a larger version of this image. Illustration: NSF

    Here on earth, buried beneath more than a kilometer of ice in the Antarctic, is a vast array of digital optical sensors known as IceCube. It’s a very special kind of telescope designed to detect neutrinos, which are one of the products — perhaps — of WIMP annihilation.

    In the convergence of these two things may lie answers, or least insights, into one of the great questions confronting 21st century astrophysics: What is dark matter? What’s the nature of this matter, which we can’t see, that makes up about five-sixths of the universe’s mass?

    Watch a YouTube video on dark matter by the Texas Cosmology Center.

    “Mainly we know what it’s not,” said Pearl Sandick, a postdoctoral fellow in the theory group of Physics Professor Steven Weinberg. “It’s not any of the particles in the Standard Model. It’s not protons or neutrons or electrons. The best guess is that most of the dark matter in the universe is made up of some particle we have yet to discover.”

    The leading candidate, Sandick said, is a WIMP, or weakly interacting massive particle. The existence of WIMPs is predicted by many theories of particle physics beyond the Standard Model. One example is supersymmetry.

    “Supersymmetry proposes a whole slew of new particles, which are the super-partners of the standard particles we know about, and one of them is going to be the lightest,” she said. “It just happens to turn out that in most supersymmetric theories, the lightest particle has roughly the properties that a dark matter particle should have.”

    The problem, however, is that if WIMPs do indeed exist, they would be incredibly hard to see. (If they were easy to see, they wouldn’t be so “dark.”) They would be electrically neutral and would have very weak interactions with normal matter. For decades, experiments have been searching for collisions of WIMPs with normal matter, yet there is no conclusive evidence that any such collisions have been observed. WIMPs are also expected to annihilate with each other, but annihilations of the WIMPs that make up the dark matter halo in which our galaxy resides have also not yet conclusively been observed.

    Photo of Pearl Sandick
    Pearl Sandick

    In order to see WIMPs, or see evidence of them, physicists have had to turn to fairly elaborate means. For example, physicists are currently trying to produce WIMPs in the Large Hadron Collider (LHC) in Switzerland by smashing together protons at energies high enough that WIMPs might be produced in the collisions. Although the newborn WIMPs wouldn’t be directly detectable at the LHC, the hope is that their existence could be inferred by the signatures of other particles produced in the same collision.

    Sandick and her collaborators are taking a more indirect — and much longer distance — approach. They propose using the IceCube neutrino detector to look at dwarf spheroidal galaxies at the margins of the Milky Way. (Read the proposal paper.) And they’re looking not for WIMPs directly, but for signs of the last ricochet in a kind of intergalactic billiards shot that began with WIMPs crashing into each other and annihilating.

    “In the early universe, WIMPs annihilated a lot,” Sandick said. “Today, the density of WIMPs is much lower, so they less frequently find each other to annihilate. In some regions of the universe, however, the density might be high enough. In the center of the sun, for instance, or in these galaxies, they might still be annihilating. So basically what we’re looking for are the annihilation products.”

    One product of the annihilation of those WIMPS, Sandick said, might be neutrinos. Those neutrinos then travel across space, pass through the earth and, finally, hit the ice where IceCube is deployed. Although neutrinos typically just pass through matter, from time to time one will collide with a particle in the ice and produce another kind of subatomic particle, a muon. And if that muon continues on through the ice at a speed faster than the speed of light in the ice, the superluminal muon will create a cone of electromagnetic radiation known as Cherenkov radiation (much like a supersonic aircraft generates a sonic boom). And it’s photons from that radiation that — at last — the IceCube sensors can register.

    “That’s the signature we’re looking for,” Sandick said, “and when you see the radiation cone, you can infer the direction the neutrino came from.”

    Even if IceCube is able to “see” these neutrinos, and differentiate them from neutrinos that might come from other sources, and reverse engineer their trajectory back to the dwarf galaxies, it wouldn’t prove that WIMPs exist. It would, however, be solid evidence in that direction, and a foundation for more experiments and more refined theory. And it would be a testament to the extraordinary ingenuity of particle physicists, as well as to their rather eccentric eagerness to cast skyhooks out into the unseen.

    This story was originally published on the Texas Science Web site.

    • Quote 2
      George Braun said on April 6, 2011 at 5:07 p.m.
      Beautiful
    • Quote 2
      Joel said on March 22, 2011 at 4:03 p.m.
      Is the density of WIMP's that low? The last quote I heard was that 23% of the universe is expected to be dark matter, 73% dark energy, and 4% baryonic matter(Scientific American Nov. 2010). Putting this together with the fact that the probable LOWEST density of baryonic matter in empty space is about 1 hydrogen per 16 cubic meters, dark matter could than be supposed at least more dense than this in most areas of the universe. I think the problem is not that they aren't annihilating but that they might not all. An observation (semi-recent, 2008) of a bullet cluster collision (via Hubble) showed that when huge droves of dark matter collide they simply slide past one another as if unaffected by its counterpart and (not surprisingly) the collision itself. Each clump followed the trajectory the cluster would have had the collision not taken place and as if no collision took place amongst the dark matter clumps themselves. If density is the criterion for annihilation of WIMP's and they do compose dark matter this should have been a prime time for observation (the scientific paper on this collision even states dark matter is most likely collisionless). If this is a sound conclusion than production from high energy conditions may be the best bet(such energy would have to be much greater than that produced LHC however, as they have yet to detect traces of dark matter production), otherwise it's to the drawing boards.
    • Quote 2
      Mike Baloka Camini said on Dec. 9, 2010 at 11:52 a.m.
      sometimes ignore diseases. The science not kill the blue planet? Thank for your answer
    • Quote 2
      Damon said on Dec. 5, 2010 at 2:48 p.m.
      OK I guess that the single other poster and myself have been ID'd as geeks!
    • Quote 2
      Damon said on Dec. 5, 2010 at 2:47 p.m.
      This reminds me of a project to detect nutrinos deep within a mine with a giant aluminium bar. Is this a better option?
    • Quote 2
      Melody (Fisher) Simon said on Dec. 2, 2010 at 10:03 p.m.
      Fascinating!
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