Santa Clara University

Professor Young (right) and her students work with the Cryogenic Dark Matter Search project—a collaboration of a dozen U.S. universities and national laboratories.

Cryogenic Detectors Research

Detecting the Existence of Dark Matter

Collaborating with others from a group of universities and national laboratories, a Santa Clara University scientist and her students are helping design, build, and conduct a grand experiment deep under the Earth that many hope will result in the first verifiable direct detection of dark matter.

Somehow during the last century, the vocabulary of physics took a fanciful bent. Consider the names given to new particles—for example, the quark, inspired by a  passage from James Joyce’s Finnegans Wake. Now physicists are in hot  pursuit of weakly interacting massive particles, or WIMPs, untold numbers of which pass right through us and the Earth every second. It is thought that they may constitute the bulk of the mysterious dark matter that—along with the even more mysterious dark energy—accounts for 95 percent of the universe’s mass, according to current cosmological models.

In the thick of the WIMP hunt are Santa Clara University physicist Betty Young and several of her brightest undergraduate students. Young serves as the University’s representative on the Cryogenic Dark Matter Search (CDMS) project. A collaboration of a dozen U.S. universities and national laboratories, the CDMS team has installed super-sensitive cryogenic particle detectors that Young helped develop for detecting WIMPs deep underground in the Soudan mine in Minnesota.

Separating Signals from Noise
The CDMS collaboration faces two daunting challenges. The first is to detect and measure the extremely faint effects following a rare collision of dark matter with ordinary matter. The second is to screen out or account for other distracting background events—due to cosmic rays, for example, which pass right through hundreds of feet of earth—so that the signatures or signals identifying actual WIMPs will stand out.

That is why this second phase of the project has been located in a deep mine. (An earlier phase, in operation at the Stanford Underground

“The Engineering Physics Program is probably one of the biggest advantages that sets Santa Clara apart in the study of physics.”

—Betty Young associate professor of physics and former Clare Booth Luce professor

Facility since the mid-1990s, has produced some of the strongest limits yet on WIMP interactions with matter.) By staging CDMS II far enough underground to escape most cosmic rays and their by-products, superfluous events can be minimized. Detection and measurement of the faint interactions themselves, however, have required the development of a whole new generation of sensors.

This has suited Young, an associate professor of physics, just fine. While earning her doctorate in physics at Stanford, she did considerable work on superconductor- and semiconductor-based detector technologies for use in cryogenic environments (near absolute zero degrees Kelvin), so she has a lot to contribute to the CDMS project.

Breeding Physicists
Working on the design and fabrication of leading-edge sensors is challenging and exciting, according to Young. “You get pulled into the field because it’s such a great way to bring together different fields, from physics to multichannel electronics to materials science and chemistry,” she says.

The key equipment installed in the Soudan mine consists of arrays of detectors that resemble hockey pucks, contained within cryogenic chambers. Made of extremely pure germanium and silicon crystals, the detectors have their top and bottom surfaces photolithographically patterned with thin films of superconducting aluminum and tungsten. In marrying the superconductor and semiconductor technologies, the researchers have created devices that are exquisitely sensitive both to the electrical charges and to the faint crystal vibrations produced by a WIMP-nucleus  collision within the cold germanium or silicon crystal lattice.

Santa Clara’s Physics Department nurtures a close-knit community of approximately 20 to 25 students in physics or engineering physics, a special program in the College of Arts and Sciences, coordinated by Young. “It’s probably one of the biggest advantages that sets Santa Clara apart in physics,” Young says. Some of her students have worked on specific projects related to the CDMS collaboration, while others have worked with her or others, both on and off campus, on a variety of other interesting research projects.

Working on cryo-detector projects is great for students studying physics or engineering physics, Young says. “It exposes them to a wide range of laboratory techniques and involves them in meaningful research related to various aspects of fundamental physics.”

Advancing Knowledge
There are at least 10 other major searches for dark matter happening internationally at this time. The competition is intense—those who are first with the proof of their discovery may well be nominated for a Nobel Prize. And yet, what if, after all this work, the identification of dark  matter particles still eludes us?

Young is professionally philosophical about outcomes. If the CDMS collaboration is unable to identify dark matter particles conclusively, their efforts will still have borne much fruit. “It would take a couple pages to list all the contributions to physics that the collaboration’s work has brought,” she says. “The amount of new technology spinning off from what we’re doing is wonderful, independent of the final dark matter result.”