Physicist takes the hunt for dark matter deep underground into a Minnesota mine

by Mark Nickel

Something about the universe doesn't add up.

Way out there in deep space, where there seems to be less stuff, the rotational speed of galaxies should slow a bit if Newton and Einstein were on the right track. But it doesn't. The speed appears to remain constant.

Either much of modern physics is wrong, or there is more stuff - a lot more stuff - than we are able to see.

Most physicists now believe that what we are able to see accounts for about 0.5 percent of matter in the universe. The rest consists of dark matter - so called because it does not react with light, has no charge and rarely interacts with luminous matter. A favored explanation for the majority of this dark matter is that it consists of new particles which are so weakly interacting that they could pass through a wall of lead stretching from Earth to the sun and not hit anything.

The search is on to document the effects of dark matter and perhaps to detect an actual particle. The numbers can be deliriously large.

In the summer of 2003, for example, astronomers at the University of California-Irvine used an X-ray telescope to observe the effects of a gigantic halo of dark matter surrounding a cluster of galaxies about a billion light-years - six billion trillion miles - from Earth. They calculated that the halo of dark matter must be something like 100 trillion times more massive than our sun.

Richard Gaitskell, assistant professor of physics at Brown (left), would settle for a lot less a lot closer to home. He is Brown's representative on the Cryogenic Dark Matter Search - CDMSII, a multi-institutional effort to detect a dark matter particle. His hunt for this weakly interacting massive particle - known by the acronym WIMP - takes him half a mile below the earth's surface in an historic iron mine in northern Minnesota.

"By running our detectors far below the earth's surface, we dramatically reduce the radiation from cosmic rays, which represent background noise," Gaitskell said. "Our experiment is now the world's most sensitive detection facility and we expect to continue to make significant improvements in the sensitivity in the next few years."

Even with half a mile of insulation above them, researchers must still contend with the earth's natural radiation, including radon gas, and with the radiation exuded by human researchers themselves. "A single drop of perspiration contains enough radiation to disturb our detectors," Gaitskell said. The detectors are prepared under clean-room conditions.

At the heart of the CDMSII detector is a "hockey puck" of pure silicon or germanium about three inches in diameter and half an inch thick (left). Six of these are stacked with their wiring to make a detector about the size and shape of a can of tennis balls. Each of several detectors is placed at the center of a series of concentric copper cylinders inside a lead-lined, polyethylene-clad circular apparatus about six feet across. When a search is in progress, the detectors are cooled to within a fraction of a degree of absolute zero, and the team waits.

All sorts of particles - including, presumably, dark matter - stream through the detectors, but the instrument is designed to distinguish what could be a WIMP from everything else.

"A WIMP that registers in our detector would do so in a very specific way," Gaitskell said. "Most electromagnetic background particles actually interact with the cloud of electrons around the nucleus of a silicon atom. The WIMP, having no charge, does not interact with electrons; it bounces off the nucleus itself. We can distinguish a nuclear recoil from an electron recoil, and we can do so with exquisite precision."

Part of that capability is a function of the very low temperature. Near absolute zero, even something as small as a single nuclear recoil would raise the temperature of the silicon puck; the detector can measure changes as small as a few millionths of a degree. But the team can also make another measurement.

"When a particle interaction occurs, it starts sending out vibrations - shock waves - in the lattice of silicon or germanium," Gaitskell said. "So we put superconducting microelectronics directly on the surface of the silicon or germanium. When the shock wave rises to the surface, the microelectronics read it out immediately - in millionths of a second. It's almost like listening to the collision."

CDMSII researchers can compare the detector's two readings of an event and develop profiles for the various particles that hit the detector. By graphing those values, they can rule out many of the events the detectors report. "Anything above or to the right of this line is essentially ruled out as having a higher cross-section or higher mass than the particles weÕd expect," Gaitskell said, drawing a sample plot on the white board.

So far, CDMSII's results, announced last May at the American Physical Society meeting, have served to force that line farther down the graph - in effect, ruling out more events and narrowing the band in which the WIMP might ultimately be found.

"It may be difficult to understand why physicists can be as excited about negative results as about positive ones," Gaitskell said, "but this data shows our experiment is working well and is in a leading position to make a clear observation of dark matter events when the necessary sensitivity is reached."

Gaitskell and his group are at work improving the sensitivity of their detectors, continuing to narrow the search. Other international groups are doing the same; the stakes are fairly high.

"The first group to unambiguously identify WIMPs is off to Stockholm," Gaitskell said. "Not only will they have solved the dark matter problem - it's been around since the 1930s, when Fritz Zwicke observed that the movements of galaxies in the Coma Cluster was not consistent with the amount of matter he could see - but they will have discovered the first evidence for a new particle beyond the standard model of particle physics. They'd have killed two birds with one stone."