Batavia, Ill.-Using detectors chilled to near absolute zero, from a vantage
point half a mile below ground, physicists of the Cryogenic Dark Matter
Search today (November 12) announced the launch of a quest that could lead
to solving two mysteries that may turn out to be one and the same: the
identity of the dark matter that pervades the universe, and the existence of
supersymmetric particles predicted by particle physics theory. Scientists of
CDMS II, an experiment managed by the Department of Energy’s Fermi National
Accelerator Laboratory hope to discover WIMPs, or weakly interacting massive
particles, the leading candidates for the constituents of dark matter-which
may be identical to neutralinos, undiscovered particles predicted by the
theory of supersymmetry.

“There’s this arrow from particle physics and this arrow from cosmology and
they seem to be pointing to the same place,” said Case Western Reserve
University’s Dan Akerib, deputy project manager of CDMS II. “Detection of a
neutralino would be very big for cosmology and it would also be very big for
particle physics.”

The CDMS II experiment, a collaboration of scientists from 12 institutions
with support from DOE’s Office of Science and the National Science
Foundation, uses a detector located deep underground in the historic Soudan
Iron Mine in northeastern Minnesota. Experimenters seek signals of WIMPs,
particles much more massive than a proton but interacting so weakly with
other particles that thousands would pass through a human body each second
without leaving a trace.

Remarkably, in the kind of convergence that gets physicists’ attention, the
characteristics of this cosmic missing matter particle now appear to match
those of the supersymmetric neutralino.

“Either that is a cosmic coincidence, or the universe is telling us
something,” said Fermilab’s Dan Bauer, CDMS project manager.

By watching how galaxies spin-how gravity affects their contingent
stars-astronomers have known for 70 years that the matter we see cannot
constitute all the matter in the universe. If it did, galaxies would fly
apart. Recent calculations indicate that ordinary matter containing atoms
makes up only 4 percent of the energy-matter content of the universe. “Dark
energy” makes up 73 percent, and an unknown form of dark matter makes up the
last 23 percent.

“It is often said that this is the ultimate Copernican Revolution,” said
David Caldwell, a physicist at the University of California at Santa Barbara
and chair of the CDMS Executive Committee. “Not only are we not at the
center of the universe, but we are not even made of the same stuff as most
of the universe.”

Measurements of the cosmic microwave background, residual radiation left
over from the Big Bang, have recently placed severe constraints on the
nature and amount of dark matter. The lightweight neutrino can account for
only a few percent of the missing mass. If neutrinos constituted the main
component of dark matter, they would act on the cosmic microwave background
of the universe in ways that the recent Wilkinson Microwave Anisotropy Probe
should have observed-but did not.

Meanwhile, particle physicists have kept a lookout for particles that will
extend the Standard Model, the theory of fundamental particles and forces.
Supersymmetry, a theory that takes a big step toward the unification of all
of the forces of nature, predicts that every matter particle has a massive
supersymmetric counterpart. No one has yet seen one of these
“superpartners.” Theory specifies the neutralino as the lightest neutral
superpartner, and the most stable, a necessary attribute for dark matter.
The neutralino’s predicted abundance and rate of interaction also make it a
likely dark matter candidate, and Caldwell noted the impact that CDMS II
could have.

“Discovery,” he said, “would be a great breakthrough, one of the most
important of the century.”

Only occasionally would a WIMP hit the nucleus of a terrestrial atom, and
the constant background “noise” from more mundane particle events-such as
the common cosmic rays constantly showering the earth-would normally drown
out these rare interactions. Placing the CDMS II detector beneath 740 meters
of earth screens out most particle noise from cosmic rays. Chilling the
detector to 50 thousandths of a degree above absolute zero reduces
background thermal energy to allow detection of individual particle
collisions. Fermilab’s Bauer estimates that with sufficiently low
backgrounds, CDMS needs only a few interactions to make a strong claim for
detection of WIMPs.

“The powerful technology we deploy allows an unambiguous identification of
events in the crystals caused by any new form of matter,” said CDMS
cospokesperson Bernard Sadoulet of the University of California at Berkeley.

Cospokesperson Blas Cabrera of Stanford University concurred.

“We believe we have the best apparatus in the world in terms of being able
to identify WIMPs,” Cabrera said.

“This endeavor is a good example of cooperation between the DOE’s Office of
High Energy Physics and the National Science Foundation in helping
scientists address the origin of the dark matter in the universe,” said
Raymond Orbach, Director of the Department of Energy’s Office of Science.

“CDMS II is the kind of innovative and pathbreaking research NSF is proud to
support,” said Michael Turner, Assistant Director for Math and Physical
Sciences at the National Science Foundation. “If it detects a signal it may
tell us what the dark matter is and give us an important clue as to how
gravity fits together with the other forces. This type of experiment shows
how the universe can be used as a laboratory for getting at the some of the
most basic questions we can ask as well as how DOE and NSF are working

While CDMS II watches for WIMPs, scientists at Fermilab’s Tevatron particle
accelerator will try to create neutralinos by smashing protons and
antiprotons together.

“CDMS can tell us the mass and interaction rate of the WIMP,” said
collaborator Roger Dixon of Fermilab. “But it will take an accelerator to
tell us whether it’s a neutralino.”

CDMS II collaborators include Brown University, Case Western Reserve
University, Fermi National Accelerator Laboratory, Lawrence Berkeley
National Accelerator Laboratory, National Institute of Standards and
Technology, Princeton University, Santa Clara University, Stanford
University, University of California at Berkeley, University of California
at Santa Barbara, University of Colorado at Denver, University of Minnesota.

Funding for the CDMS II experiment comes from the Office of Science of the
U.S. Department of Energy and the Astronomy and Physics Division of the
National Science Foundation.

Fermilab is a national laboratory funded by the Office of Science of the
U.S. Department of Energy and operated by Universities Research Association,

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