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New
cryogenic detectors probe recent evidence for dark matter particle
29
Feb 2000
IN THIS STORY:
Opposite
results also reported 
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A
new generation of particle detector that operates at temperatures
near absolute zero has proven extremely accurate in identifying
the particles that crash through it, an international team of
scientists reported last week.
The
novel detector, buried 35 feet underground on the Stanford University
campus, has dedicated itself for more than a year to the search
for exotic and elusive particles that, according to some theories,
make up more than 90 percent of the mass of the universe.
Though
the device has yet to find evidence of such particles - known
collectively as WIMPs, or weakly interacting massive particles
- it has proved to have a keen ability to discriminate between
the different kinds of particles that zip through it.
Developed
over the past 11 years by scientists at 10 institutions and
coordinated by the Center for Particle Astrophysics at the University
of California, Berkeley, it uses germanium or silicon semiconductors
cooled to several hundredths of a degree above absolute zero,
the coldest possible temperature. The scientific team goes by
the name Cryogenic Dark Matter Search, or CDMS.
One
of the more common particle detectors today relies upon sodium
iodide crystals that give off a burst of light when a particle
passees through it. When a particle passes through the new semiconductor
detector, the detector is both ionized and heated. Measurement
of both allows much better discrimination and identification
of particles.
UC
Berkeley assistant research physicist Rick Gaitskell reported
the consortium's results and conclusions Feb. 25 at the Fourth
International Symposium on Sources and Detection of Dark Matter
in the Universe in Marina del Rey, Calif.
Opposite results also reported
Another
group of researchers based in Rome and Beijing and calling themselves
the DArk MAtter (DAMA) group, reported just the opposite at
the same meeting. Using a detector containing 100 kilograms
of sodium iodide, they claim to see evidence for the existence
of a neutralino, a type of WIMP predicted by the most popular
theory of particle physics, supersymmetry.
"Given
our own results and our level of sensitivity, I have to disagree
with the interpretation of the DAMA results in terms of Weakly
Interacting Massive Particles," said Bernard Sadoulet, a member
of the CDMS team. Sadoulet, a professor of physics at UC Berkeley,
is director of the Center for Particle Astrophysics and has
worked for more than 15 years on WIMP searches.
"True,
since we are using different target materials we both could
be right, but within the currently favored theoretical framework
our results appear seriously incompatible. We are simply not
observing enough events in CDMS above our neutron contamination,
and we have found no experimental effect that could account
for this."
The
American team's novel detector is comparable in sensitivity
to the sodium iodide detector of the Italian group, but is superior
in its ability to distinguish background "events" - interactions
in the detector that result from known particles - from likely
dark matter interactions.
"The
discrimination capability of these detectors is amazing," said
Sunil Golwala, a UC Berkeley graduate student who did a large
fraction of the work reported this week. "We're literally looking
for a needle in a haystack. We start with 6.4 million events
and end with 13 events that are of interest for dark matter
searches. And these 13 events are, in fact, what we expect based
on our simulations of the neutron background in our facility."
"What
is most exciting to me is that we've been able to employ a new
type of detector, developed explicitly to do this type of measurement,
to obtain a result with implications for a fundamental question
in cosmology," said Tony Spadafora, associate director of CfPA.
"These measurements are difficult and take a long time because
WIMP signals are thought be very small and infrequent."
Their
findings have been submitted to Physical
Review Letters.
Source:
Robert Sanders, Public Affairs
RELATED STORIES, SITES, PHOTOS:
Full
press release
Center
for Particle Astrophysics (CfPA)
NY
Times article about the new results
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