LOS ANGELES (2/29/2000) -- 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 throught 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.
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 the University of California, 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".
The CDMS collaboration is supported jointly by the Department of Energy and by the National Science Foundation in a collaboration that includes groups from UC Berkeley, Stanford, UC Santa Barbara, Lawrence Berkeley National Laboratory, Fermi National Accelerator Laboratory (Fermilab), Case Western Reserve University, Santa Clara University, the National Institute of Standards and Technology in Boulder, Co., the University of Colorado at Denver and Princeton University.
Their findings have been submitted to Physical Review Letters, and are available at http://arXiv.org/abs/astro-ph/?0002471.
For more than a decade, various experiments around the world have searched for dark matter in the form of WIMPs. Theoretically, a million WIMPs would pass through an area the size of a thumbnail each second, but only about one per day would interact in a one-kilogram germanium detector.
Detectors capable of recording one such event per day per kilogram, or less, are sufficiently sensitive to search for a particular type of WIMP suggested by supersymmetry - the neutralino.
Last week, the DAMA collaboration reported on more than three years of data collected with a 100-kilogram sodium iodide detector operated deep underground in the Gran Sasso National Laboratory in Italy. In a statistical analysis of tens of thousands of events, they claim to see a modulation in the event rate with the same period as Earth's solar year, with a maximum in June and a minimum in December.
This annual modulation is expected to result from the motion of the sun and earth system through a massive cloud of WIMPs as our solar system rotates about the center of our galaxy.
The CDMS experiment is based on about half a kilogram of detector mass operated over one year and producing about 12 kilogram days of data.
The CDMS experiment is housed in a cave; the dirt helps shield the cryogenic detectors from cosmic radiation. Further shielding consists of lead, polyethylene and active plastic scintillator, which produces a tiny light flash for every particle interaction. These allow the influence of cosmic and terrestrial radiation to be reduced by a factor of about 10,000.
The only particles that can get through and mimic WIMPs such as the neutralino are neutrons, which also bounce off atomic nuclei in the detectors. During its one-year run, CDMS observed 13 single scattering nuclear recoils, all of which were identified as neutrons and not WIMPs.
"The method used by DAMA is both very clever and very demanding," Sadoulet said. "The rate variation that they are measuring over the year is about one percent deviation from the mean, and it is extremely difficult to guarantee that it is not due to instrumental effects. Our Italians colleagues have tried their best and so far have found no other explanation than a WIMP signal. Given the significance that such an interpretation would have, it is very important to keep looking for more mundane explanations. Many things vary seasonally, for instance the temperature of the laboratory, the level of water in the Gran Sasso mountain, which could influence in subtle ways the response of the DAMA apparatus. To be frank, although I have not enough information to identify the possible cause, I am not totally convinced by the DAMA arguments claiming to rule out such effects. "
These reports are not the end of the story, the researchers say. CDMS and DAMA both plan expanded new experiments. DAMA will increase its detector mass from 100 to 250 kilograms, and the newly approved CDMS-II experiment will move deep underground to the Soudan mine in northern Minnesota. CDMS-II will utilize more than 10 times the present detector mass in an environment where the neutron background has been reduced by nearly a factor of 1,000.
"We are beginning to probe interesting territory for WIMPs and are looking forward to a factor of 100 increase of sensitivity with our second-generation experiment," Spadafora said.
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