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Posted by Andrew Yee on January 4, 2007, 6:09 pm
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News Service
Stanford University
Stanford, California
Contact:
Dawn Levy, News Service
(650) 725-1944
Comment:
Blas Cabrera, Physics
(650) 723-3395
November 28, 2006
Dark matter hides, physicists seek
By Clara Moskowitz
Scientists don't know what dark matter is, but they know it's all over the
universe. Everything humans observe in the heavens -- galaxies, stars,
planets and the rest -- makes up only 4 percent of the universe, scientists
say. The remaining 96 percent is composed of dark matter and its even more
mysterious sibling, dark energy. Scientists recently found direct evidence
that dark matter exists by studying a distant galaxy cluster and observing
different types of motion in luminous versus dark matter. Still, no one
knows what dark matter is made of. Now, a pioneering international project
co-led by Stanford physicist Blas Cabrera may finally crack the case and pin
down the elusive particles that form dark matter.
"It's harder and harder to get away from the fact that there is a substance
out there that's making up most of the universe that we can't see," says
Cabrera. "The stars and galaxies themselves are like Christmas tree lights
on this huge ship that's dark and neither absorbs nor emits light."
Buried deep underground in a mineshaft in Minnesota lies Cabrera's project,
called the Cryogenic Dark Matter Search II (CDMS II). University of
California-Berkeley physicist Bernard Sadoulet serves as spokesperson for
the effort. Fermilab's Dan Bauer is its project manager, and Dan Akerib from
Case Western Reserve University is the deputy project manager. A team of 46
scientists at 13 institutions collaborates on the project.
To catch a WIMP
The experiment is the most sensitive in the world aiming to detect exotic
particles called WIMPS (Weakly Interacting Massive Particles), which are one
of scientists' best guesses at what makes up dark matter. Other options
include neutrinos, theorized particles called axions or even normal matter
like black holes and brown dwarf stars that are just too faint to see.
WIMPS are thought to be neutral in charge and weigh more than 100 times the
mass of a proton. At the moment these elementary particles exist only in
theory and have never been observed. Scientists think they haven't found
them yet because they're excruciatingly difficult to capture. WIMPS don't
interact with most matter -- the shy particles pass right through our bodies
-- but CDMS II aims to catch them in a rare collision with the atoms in the
project's special-made detectors.
"These particles mostly pass through the Earth without scattering," Cabrera
says. "The only reason we even have a chance of seeing events is because
[there are] so many of the particles that very rarely one will come [into
the detector] and scatter."
The detectors are hidden under layers of earth in Minnesota's Soudan mine to
protect them from cosmic rays and other particles that might collide with
the detectors and be mistaken for dark matter. In fact, half the battle for
the scientists working on CDMS II is to shield their instruments as much as
possible from everything but WIMPS and to develop elaborate systems to tell
the difference between dark matter and more mundane particles.
"Our detector is this hockey-puck-shaped thing that needs to live at 50
thousandths of a degree above absolute zero," says Walter Ogburn, a graduate
student at Stanford who works on the project. "It's hard to make things that
cold."
To that end, the instruments are nestled in a canister called an icebox,
lined with six layers of insulation, from room temperature on the outside to
coldest on the inside. This keeps the detectors so cold that even atoms
can't shiver.
The detectors are made of crystals of solid silicon and solid germanium. The
silicon or germanium atoms sit still in a perfect lattice. If WIMPS crash
into them, they will wiggle and give off tiny packets of heat called
phonons. When phonons rise to the surface of the detectors, they create a
change in a very sensitive layer of tungsten, which the researchers can
record. A second circuit on the other side of the detector measures ions,
charged particles that would be released from a collision of a WIMP and an
atom in the detector.
"Those two channels let us discriminate between different kinds of
interactions," says Ogburn. "Some things make more ionization and some
things make less, so you can tell the difference that way."
It takes a squad of scientists at multiple facilities to build the
detectors. The team buys the crystals from an outside company, and
researchers at Stanford's Center for Integrated Systems make measuring
instruments on the surfaces of the detectors. "We use the same things to
make these that people use to make microprocessors because those are also
super tiny," says Matt Pyle, another graduate student in Cabrera's lab.
Clumps of clues
A subset of WIMPS, called neutralinos, are the lightest particles expected
by supersymmetry, a theory that predicts a mate for every particle we've
already observed. If CDMS II is successful in finding neutralinos, this
would be the first evidence for supersymmetry. "Supersymmetry suggests
there's a whole other sector out there of particles that are the partners to
our existing particles," Cabrera says. "There are many ways in which
supersymmetry looks very likely. But there's no direct evidence yet for any
matching [supersymmetric] particle pair."
The weak interactions of WIMPS are why, even though dark matter particles
have mass and obey the laws of gravity, they do not clump into galaxies and
stars like normal matter. In order to clump, particles must crash and stick
together. But WIMPS most often would fly right by each other. Plus, because
WIMPS are neutral, they do not form atoms, which require the attraction of
positively charged protons to negatively charged electrons.
"Dark matter permeates everything," Cabrera says. "It just never collapsed
the way atoms did."
Since dark matter never formed stars and other familiar heavenly objects,
for a long time scientists never knew it was there. The earliest indication
of its existence came in the 1930s when Fritz Zwicky, a Swiss-American
astronomer, observed clusters of galaxies. He added up the masses of
galaxies and noticed that there was not enough mass to account for the
gravity that must exist to hold the clusters together. Something else must
provide the missing mass, he deduced.
Later in the 1970s, Vera Rubin, an American astronomer, measured the speeds
of stars in the Milky Way and other nearby galaxies. As she looked farther
out toward the edges of these galaxies, she found that the stars do not
rotate more slowly as scientists expected. "That didn't make any sense,"
Cabrera says. "The only way you could understand it is if there was a lot
more mass there than what you saw in the starlight."
Over the years, more and more evidence for dark matter has piled up.
Although scientists don't yet know what it is, they have a better idea of
where it is and how much of it there should be. "There's very little wiggle
room left for having different quantities," Cabrera says.
"We've not seen anything that looks like an interesting signal to date," he
says. But the CDMS II researchers continue the search. So, too, do other
groups. ZEPLIN, an experiment run by physicists at the University of
California-Los Angeles and the United Kingdom Dark Matter Collaboration,
aims to catch WIMPs in liquid vats of xenon in a mine near Sheffield,
England. And at the South Pole, a University of Wisconsin-Madison project
called IceCube is under construction that will use optical sensors buried
deep in the ice to look for neutrinos, high-energy particles that are
signatures of WIMP annihilations.
Meanwhile, CDMS II continues to evolve. Its researchers are building bigger
and bigger detectors to increase their chances of finding WIMPS. In the
future, the team hopes to build a 1-ton detector that should be able to
discover many of the most probable types of WIMPS, if they exist. "We're
taking data now with more than twice as much target mass of germanium than
we had before, so we're definitely exploring new territory right now," says
Ogburn. "But there's a lot more to cover."
[Clara Moskowitz is a science-writing intern with Stanford News Service.]
-30-
Editor Note: Science-writing intern Clara Moskowitz wrote this release. A
photo of the detector is available on the web at
http://newsphotos.stanford.edu/CDMS/
Relevant Web URLs:
* Cryogenic Dark Matter Search II Website
http://cdms.berkeley.edu/
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