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New cryogenic detectors probe recent evidence for dark matter particle
29 Feb 2000

germanium detectors

Four of the six germanium detectors used in the CDMS search for WIMPs. They are stacked in the rear tower and suspended in a cryogenic container, or cryostat.


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.

cryostat image
The inner layers of the cryostat, showing the radiopure copper used to minimize the radioactive background environment for the experiment. The hexagonal holes, where the detectors are mounted, are kept at .01 degrees above absolute zero.

"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


Full press release

Center for Particle Astrophysics (CfPA)

NY Times article about the new results