Search for dark matter illuminates largest and smallest mysteries of universe
With the first data from their “underground observatory” in Northern Minnesota, scientists of the Cryogenic Dark Matter Search (CDMS II) have peered with greater sensitivity than ever before into the suspected realm of the Weakly Interacting Massive Particles (WIMPs). The sighting of WIMPs could answer the double-mystery of dark matter on the cosmic scale of astrophysics, and of supersymmetry on the subatomic scale of particle physics.
The CDMS II result, described in a paper submitted to Physical Review Letters and presented at a Denver meeting of the American Physical Society May 3-4, shows with 90 percent certainty that the interaction rate of a WIMP with mass 60 GeV must be about one interaction every 25 days per kilogram of germanium (detector material). The measurements from the CDMS II detectors are at least four times more sensitive than the best previous measurement offered by the EDELWEISS experiment, a European collaboration with its underground laboratory near Grenoble, France.
“Think of this improved sensitivity like a new larger telescope with twice the diameter and thus four times the light collection of any that came before it,” said Blas Cabrera, CDMS II co-spokesperson and a Stanford professor of physics. “We are now able to look for a signal that is just one-fourth as bright as any we have seen before. Over the next few years, we expect to improve our sensitivity with CDMS II detectors by a factor of 20 or more.”
Said CDMS II co-spokesperson Bernard Sadoulet of the University of California-Berkeley: “We know that neither our Standard Model of particle physics nor our model of the cosmos is complete. This particular missing piece seems to fit both puzzles, in particle physics and in astrophysics. We are seeing the same shape from two different directions.”
WIMPs, which carry no charge, are a study in contradictions. While physicists expect them to have about 100 times the mass of protons, their ghostly nature allows them to slip through ordinary matter while leaving barely a trace. The term “weakly interacting” refers not to the amount of energy deposited when they interact with normal matter, but rather to the fact that they interact extremely infrequently. In fact, as many as 100 billion WIMPs may have been streaming through your body undetected while you have been reading these first few sentences.
Ganging up to detect WIMPs
With 48 scientists from 13 institutions, plus another 28 engineering, technical and administrative staffers, CDMS II operates with funding from the Office of Science of the U.S. Department of Energy (DOE), from the Astronomy and Physics Divisions of the National Science Foundation (NSF) and from member institutions. The DOE’s Fermi National Accelerator Laboratory (Fermilab) provides the project management for CDMS II.
“The nature of dark matter is fundamental to our understanding of the formation and evolution of the universe,” said Raymond L. Orbach, director of DOE’s Office of Science.
Michael Turner, assistant director for math and physical sciences at NSF, described “identifying the constituent of the dark matter” as “one of the great challenges in both astrophysics and particle physics.”
“Dark matter holds together all structures in the universe – including our own Milky Way – and we still do not know what the dark matter is made of,” Turner said. “The working hypothesis is that it is a new form of matter – which if correct will shed light on the inner workings of the elementary forces and particles. In pursuing the solution to this important puzzle, CDMS is now at the head of the pack, with another factor of 20 in sensitivity still to come.”
The presence of dark matter in the universe is detected through its gravitational effects on all cosmic scales, from the growth of structure in the early universe to the stability of galaxies today. Most astrophysicists believe that this unseen “dark matter” cannot be made of the ordinary matter forming the stars, planets and other objects in the visible universe. Cosmological observations have determined that dark matter constitutes as much as seven times more total mass than ordinary matter. WIMPs produced in the early universe are a major contender for this mysterious component.
“Something out there formed the galaxies and holds them together today, and it neither emits nor absorbs light,” said Cabrera. “The mass of the stars in a galaxy is only 10 percent of the mass of the entire galaxy, so the stars are like Christmas tree lights decorating the living room of a large, dark house.”
Physicists also believe WIMPs could be the as-yet unobserved subatomic particles called neutralinos. These would be evidence for the theory of supersymmetry, introducing intriguing new physics beyond today’s Standard Model of fundamental particles and forces.
Supersymmetry predicts that every known particle has a supersymmetric partner with complementary properties, although none of these partners has yet been observed. However, many models of supersymmetry predict that the lightest supersymmetric particle, called the neutralino, has a mass about 100 times that of the proton.
“Theorists came up with all of these so-called ‘supersymmetric partners’ of the known particles to explain problems on the tiniest distance scales,” said Dan Akerib of Case Western Reserve University. “In one of those fascinating connections of the very large and the very small, the lightest of these superpartners could be the missing piece of the puzzle for explaining what we observe on the very largest distance scales.”
Experiment in a mine
The CDMS II team practices “underground astronomy,” with particle detectors located nearly a half-mile below the Earth’s surface in a former iron mine in Soudan, Minn. The 2,341 feet of the Earth’s crust shields out cosmic rays and the background particles produced by them. The detectors are made of germanium and of silicon, important semiconductor crystals with similar properties (germanium is two and half times as dense as silicon). The detectors are chilled to within one-tenth of a degree of absolute zero, a temperature where molecular motion becomes negligible. These unique detectors simultaneously measure both the charge and vibration produced by particle interactions within the crystals. WIMPS will signal their presence by releasing less charge than most background particles produce for the same amount of vibration.
“Our detectors act like a telescope equipped with filters that allow astronomers to distinguish one color of light from another,” said CDMS II project manager Dan Bauer of Fermilab. “Only, in our case, we are trying to filter out conventional particles in favor of dark matter WIMPS.”
Physicist Earl Peterson of the University of Minnesota oversees the Soudan Underground Laboratory, also home to Fermilab’s long-baseline neutrino experiment, the Main Injector Neutrino Oscillation Search.
“I’m excited about the significant new result from CDMS II, and I congratulate the collaboration,” Peterson said. “I’m pleased that the facilities of the Soudan Laboratory contributed to the success of CDMS II. And I’m especially pleased that the work of Fermilab and the University of Minnesota in expanding the Soudan Laboratory has resulted in superb new physics.”
This new region in the search for dark matter WIMPs will be explored by the CDMS II scientists for the first time over these next few years. Many supersymmetric models predict neutralinos with just the right properties to make up the dark matter. So either the dominant mass of our universe will be discovered, or a large range of supersymmetric models will be excluded from possibility. Either way, the CDMS II experiment will play a major role in advancing our understanding of particle physics and of the cosmos.
The CDMS II collaborating institutions include Brown University, Case Western Reserve University, Fermi National Accelerator Laboratory, Lawrence Berkeley National Laboratory, the National Institutes of Standards and Technology, Princeton University, Santa Clara University, Stanford University, the University of California-Berkeley, the University of California-Santa Barbara, the University of Colorado-Denver, the University of Florida and the University of Minnesota.
Mike Perricone is an editor at Fermilab.
