Crystal bells stay silent in CDMS dark matter search

Inspired by his brother Erik's research on dark matter, musician Karl Ramberg built a musical model of the CDMS detector, in collaboration with CDMS scientists. Click on the image to watch the five-minute video on YouTube.

Scientists of the Cryogenic Dark Matter Search experiment today announced that they have regained the lead in the worldwide race to find the particles that make up dark matter. The CDMS experiment, conducted a half-mile underground in a mine in Soudan, Minn., again sets the world’s best constraints on the properties of dark matter candidates.

“With our new result we are leapfrogging the competition,” said Blas Cabrera of Stanford University, co-spokesperson of the CDMS experiment, for which Fermilab hosts the project management. “We have achieved the world’s most stringent limits on how often dark matter particles interact with ordinary matter and how heavy they are, in particular in the theoretically favored mass range of more than 40 times the proton mass. Our experiment is now sensitive enough to hear WIMPs even if they ring the ‘bells’ of our crystal germanium detector only twice a year. So far, we have heard nothing.”

WIMPs, or weakly interacting massive particles, are leading candidates for the building blocks of dark matter, which accounts for 85 percent of the entire mass of the universe. Hundreds of billions of WIMPs may have passed through your body as you read these sentences.

“We were disappointed about not seeing WIMPs this time. But the absence of background in our sample shows the power of our detectors as we enter into very interesting territory,” said CDMS co-spokesperson Bernard Sadoulet, of the University of California, Berkeley.

If they exist, WIMPs might interact with ordinary matter at rates similar to those of low-energy neutrinos, elusive subatomic particles discovered in 1956. But to account for all the dark matter in the universe and the gravitational pull it produces, WIMPs must have masses about a billion times larger than those of neutrinos. The CDMS collaboration found that if WIMPs have 100 times the mass of protons (about 100 GeV/c²) they collide with one kilogram of germanium less than a few times per year; otherwise, the CDMS experiment would have detected them.

“The nature of dark matter is one of the mysteries in particle physics and cosmology,” said Dr. Dennis Kovar, Acting Associate Director for High Energy Physics in the U.S. Department of Energy's Office of Science. “Congratulations to the CDMS collaboration for improved sensitivity and a new limit in the search for dark matter.”

The CDMS experiment is located in the Soudan Underground Laboratory, shielded from cosmic rays and other particles that could mimic the signals expected from dark matter particles. Scientists operate the ultrasensitive CDMS detectors under clean-room conditions at a temperature of about 40 millikelvin, close to absolute zero. Physicists expect that WIMPs, if they exist, travel right through ordinary matter, rarely leaving a trace. If WIMPs crossed the CDMS detector, occasionally one of the WIMPs would hit a germanium nucleus. Like a hammer hitting a bell, the collision would create vibrations of the detector's crystal grid, which scientists could detect. Not having observed such signals, the CDMS experiment set limits on the properties of WIMPs.

"Observations made with telescopes have repeatedly shown that dark matter exists. It is the stuff that holds together all cosmic structures, including our own Milky Way. The observation of WIMPs would finally reveal the underlying nature of this dark matter, which plays such a crucial role in the formation of galaxies and the evolution of our universe," said Joseph Dehmer, director of the Division of Physics for the National Science Foundation.

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Zimmer, other leaders advocate for basic science research funding

From The University of Chicago Chronicle,
Feb. 21, 2008

President Zimmer and a coalition of nearly a dozen industry and university leaders visited Capitol Hill on Tuesday, Feb. 12 to advocate funding increases for basic science research, including the National Laboratory System.

On the coalition's agenda was both a $500 million supplemental appropriation in FY2008 for the DOE Office of Science and the National Science Foundation, and for a larger FY2009 budget for the agencies.

The coalition represented the Association of American Universities and the National Association of State Universities and Land-Grant Colleges.

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Vector inflation points the way

From PhysicsWorld.com,
Feb. 20, 2008

You've got to love physics humour. To mark Valentine's day last week, Fermilab's in-house magazine ran a spoof personal ad that stated: "mature paradigm with firm observational support seeks a fundamental theory in which to be embedded". It was referring to inflation - a period of exponential expansion thought to have taken place 10-35 s after the big bang, which, although able to account for the large-scale appearance of the universe, lacks a firm theoretical footing.

By chance, that same day three theorists posted a paper on the arXiv preprint server that could help remedy this situation.

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