March 2, 2000

 
 
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Some light may soon be shed on dark matter, the invisible stuff of which most of the universe seems to be made. The Cryogenic Dark Matter Search (CDMS), a collaboration of researchers from 10 institutions including the Lawrence Berkeley National Laboratory, has announced the achievement of "the world's best discrimination in the search for dark matter." Their findings appear "incompatible" with the findings reported by another multi-institutional collaboration.

OUR MILKY WAY GALAXY APPEARS TO BE SURROUNDED BY A HALO OF GAMMA RAYS. ONE POSSIBLE CAUSE FOR THIS HIGH-ENERGY GLOW ARE WIMPS WEAKLY INTERACTING MASSIVE PARTICLES THAT WOULD BE THE PRIME CONSTITUENTS OF DARK MATTER, THE INVISIBLE STUFF OF WHICH MOST OF THE UNIVERSE SEEMS TO BE MADE.
Photo courtesy of NASA

Various astronomical observations together with more than 70 years of combined astrophysics and particle physics research have shown that perhaps as much as 90 percent of the known universe is made up of matter that is visible to us only by its gravitational effects. Since it cannot be seen by the light it emits, this matter has been dubbed "dark." The nature of dark matter holds implications for the evolution, structure, and ultimate fate of the universe, as well as for our current physics models. Consequently, astrophysicists and cosmologists have undertaken an intense search for the source of dark matter. One of the prime potential candidates is a family of weakly interacting massive particles or "WIMPS."

On February 19, the New York Times broke the story that the DArk MAtter experiment (DAMA), a collaboration based in Rome and Beijing, was reporting evidence for having found a particular type of WIMP called a "neutralino." This particle was said to be at least 50 times more massive than a proton yet so non-interactive with conventional matter that a billion might pass through your body every second without a trace.

The DAMA results were based on three years of data collected on a sodium iodide detector operated deep underground in the Gran Sasso National Laboratory in Italy. In a statistical analysis of tens of thousands of recorded events, seasonal fluctuations were observed that were attributed to the movement of the Earth either with or against the flow of a theorized massive cloud of WIMPs permeating our galaxy.

On February 25, at the Fourth International Symposium on Sources and Detection of Dark Matter in the Universe in Marina del Rey, California, the CDMS collaboration announced their own results which are based on an entirely new type of detector technology that employs crystals kept at cryogenic temperatures to detect potential dark matter particles. So far, this most discriminating of all dark matter detector systems has found no particles that could be clearly identified as WIMPs. The events they have detected have all been attributed to neutrons.

"The powerful technology we deploy derives its advantage from being able to distinguish background events that result from many of the known particles interacting in the crystals from those that are likely to be dark matter interactions," says Bernard Sadoulet, the spokesperson for CDMS, a staff scientist with Berkeley Lab's Physics Division, professor of physics at UC Berkeley, and the head of the Center for Particle Astrophysics where he has been leading the search for WIMP candidates. "This discrimination allows an unambiguous identification of events in the crystals caused by any new form of matter."

The key to the high sensitivity of the CDMS detectors are crystals of germanium or silicon cooled to within 0.1 degrees above absolute zero (the coldest possible temperature). A WIMP such as a neutralino interacting with a nucleus in these crystals will produce both heat and ionization at characteristic ratios which can be used to reject background on an event-by-event basis.

"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," says UC Berkeley physicist Tony Spadafora, the deputy project manager for CDMS who is also affiliated with our Physics Division. "These measurements are difficult and take a long time because WIMP signals are thought be very small and infrequent".

Much of the development of the CDMS detectors was done by Sunil Golwala, a UC Berkeley graduate student, working with semiconductor crystal expert Eugene Haller, of the Lab's Materials Sciences Division. The detectors are housed inside a unique cryostat, designed and built at Berkeley Lab by a team led by Ron Ross of the Lab's Physics Division and Dick Wolgast of the Engineering Division. This cryostat maintains ultra-low temperatures while helping to shield the detectors from cosmic radiation. The CDMS experiment is being carried out inside a a cave, 10.5 meters below the surface on the Stanford campus. The combination of dirt and cryostat shielding reduces the influence of cosmic and terrestrial radiation by a factor of about 10,000.

The events that do breach the shielding are primarily gamma and beta rays which produce electron recoils within the detectors. WIMPs, including neutralinos, would only interact with atomic nuclei and the CDMS detectors can tell whether a recoiling particle was an electron or a nucleus by simultaneously measuring the ratio of heat to ionization generated within the germanium or silicon crystals. After eliminating all electron recoils from their data sets, the CDMS researchers observed 13 nuclear recoils. They concluded that all were from neutrons which, like neutralinos, interact only with nuclei.

Says Sadoulet, "No rigorous statement can be made on the compatibility between our experiment and the DAMA experiment because we are using different target materials. However in the favored theoretical framework we appear to be seriously incompatible. The possibility remains though that the physics for this unknown particle is different from what we expect and that we are both right!"

The CDMS project is funded jointly by the U.S. Department of Energy and by the National Science Foundation in a collaboration coordinated by the Center for Particle Astrophysics. The next phase of the project, CDMS II, has recently been approved by DOE and NSF.

Says Spadafora, "We're starting construction on a version of our apparatus to be run in a former iron mine in northern Minnesota. This deep underground location provides much better shielding from cosmic rays which will allow us to do an even more sensitive experiment"

In addition to Berkeley Lab, UC Berkeley and Stanford, the CDMS collaboration includes groups from UC Santa Barbara, Fermilab, Case Western Reserve University, Santa Clara University, the National Institute of Standards and Technology, the University of Colorado, and Princeton.

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