CDMS experiment results covers new ground in search for dark matter

University of Minnesota researchers play a key role in the effort

Scientists looking for dark matter face a serious challenge: No one knows what dark matter particles look like. So their search covers a wide range of possible traits—different masses, different probabilities of interacting with regular matter.

The gravitational effects of dark matter are seen in the motion of stars, the bending of light and the evolution of galactic structure. With five times more dark matter than normal matter, this is not a small effect. However, to get one of these particles to interact with a detector here on Earth is very difficult since they are so weakly interacting.

Such rare events require sensitive detectors which can tell the difference between a dark matter interaction and the much more common backgrounds from natural radiation. Cosmic rays are an overwhelming background unless the experiments are shielded from them by thousands of feet of rock. Two world-leading dark matter searches are in operation half a mile underground at the University of Minnesota’s Soudan Underground Lab. The mine provides shielding from cosmic rays that could clutter the detector as it waits for passing dark matter particles.

Today, scientists on one of the experiments, namely the Cryogenic Dark Matter Search, or CDMS, announced results from a run with a new set of advanced germanium detectors. The germanium is maintained at temperatures of only 40 thousandths of a degree above absolute zero in order to detect the very small energies deposited by the collisions of dark matter particles with the germanium. 

“We’re pushing CDMS to as low mass as we can,” says Fermilab physicist Dan Bauer, the project manager for CDMS. “We’re proving the particle detector technology here.” 

New dark matter territory

Today’s result carves out interesting new dark matter territory for masses below 6 billion electronvolts. The dark matter experiment Large Underground Xenon, or LUX, recently ruled out a wide range of masses and interaction rates above that with the announcement of its first result in October 2013. University of Minnesota Professor Priscilla Cushman has been a key player in the CDMS since moving the experiment from a shallow site in California to the mine in northern Minnesota a decade ago. She and University of Minnesota Professor Vuk Mandic and their students and postdocs take turns operating the detector at the Soudan site as well as a cryogenic lab on the Twin Cities campus used to test detectors before installation. Analysis of the resulting data is a team effort across the participating institutions, with multiple video conferences each week. “We have shown that we can find dark matter particles even if they have very little mass,” says Cushman. “We will continue to push to even lower cross sections with these detectors and if dark matter is indeed composed of low mass particles, CDMS is the experiment that will find them.” Scientists have expressed an increasing amount of interest of late in the search for low-mass dark matter particles, with CDMS and three other experiments—DAMA, CoGeNT and CRESST—all finding their data compatible with the existence of dark matter particles between 5 billion and 20 billion electronvolts. But such light dark-matter particles are hard to pin down. The lower the mass of the dark-matter particles, the less energy they leave in detectors, and the more likely it is that background noise will drown out any signals.

Confounding results

Even more confounding is the fact that scientists don’t know whether dark matter particles interact in the same way in detectors built with different materials. In addition to germanium, scientists use argon, xenon, silicon and other materials to search for dark matter in more than a dozen experiments around the world.

Their result, which does not claim any hints of dark matter particles, contradicts a result announced in January by CoGeNT, the other dark matter experiment running at Soudan. CoGeNT, like CDMS, uses germanium-based detectors. This is important since dark matter particles may interact differently with different target materials. Scientists use argon, xenon, silicon and other materials to search for dark matter in more than a dozen experiments around the world. Last year CDMS itself found three events that might have been dark matter particles in a run using silicon- based detectors. 

“It’s important to look in as many materials as possible to try to understand whether dark matter interacts in this more complicated way,” says Adam Anderson, a graduate student at MIT who worked on the latest CDMS analysis as part of his thesis. “Some materials might have very weak interactions. If you only picked one, you might miss it.” 

Scientists around the world seem to be taking that advice, building different types of detectors and constantly improving their methods.

“Progress is extremely fast,” Anderson says. “The sensitivity of these experiments is increasing by an order of magnitude every few years.”

Adapted from a story in Fermilab’s Symmetry magazine by By Kathryn Jepsen