CDMS Opens the Box, Reveals Results in Search for Dark Matter

EDITOR’S NOTE: Texas A&M University’s Dr. Rupak Mahapatra, assistant professor of physics and principal investigator for the Texas A&M Cryogenic Dark Matter Search (CDMS) experiment group, will be presenting a colloquium Friday (Dec. 18) on results from the experiment’s latest search using data collected during the past two years. Mahapatra’s talk, which will broadcast via live Windows Media webcast at (Channel 8), is scheduled for 2 p.m. (CST) in the Stephen W. Hawking Auditorium, located in the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy on the Texas A&M campus. A paper detailing these results also can be found online. Mahapatra will be available for interviews concerning both his talk and the paper via telephone (979) 845-8624 or email mahapatra@physics.tamu.edu

COLLEGE STATION — In an analysis of new data, scientists from the Cryogenic Dark Matter Search (CDMS) experiment, managed by the United States Department of Energy’s Fermi National Accelerator Laboratory and involving physicists from Texas A&M University, have detected two events whose characteristics match those of the particles that physicists believe make up dark matter.

However, there is a chance that both events could be the signatures of background particles — other particles with interactions that mimic the signals of dark matter candidates. Scientists have a strict criterion when determining whether they have made a discovery. There must be fewer than one chance in 1,000 that the observed events could be due to background. This result does not yet pass that test, so CDMS experimenters do not claim to have detected dark matter.

CDMS researchers announced their result in talks around the nation Thursday and Friday, including one by Texas A&M Assistant Professor of Physics Dr. Rupak Mahapatra scheduled for 2 p.m. (CST) Friday in the Stephen W. Hawking Auditorium, located in the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy on the Texas A&M campus. The collaboration details the results in a paper, Results from the Final Exposure of the CDMS II Experiment, that they have submitted to the physics preprint archive and for publication.

Astronomical observations from telescopes, satellites and measurements of the cosmic microwave background have led scientists to believe that most of the matter in the Universe neither emits nor absorbs light. If it exists, this dark matter would have provided the gravitational scaffolding that allowed normal matter to coalesce into the galaxies we see today. In particular, scientists think our own galaxy is embedded within an enormous cloud of dark matter. As our solar system rotates around the galaxy, it moves through this cloud.

Particle physics theories suggest that dark matter is composed of Weakly Interacting Massive Particles (WIMPs). Scientists expect these particles to have masses comparable to, or perhaps heavier than, the masses of atomic nuclei. Although such WIMPs would rarely interact with normal matter, they may occasionally bounce off — or scatter like billiard balls from — an atomic nucleus, leaving behind a small amount of energy that is detectable under the right conditions.

The CDMS experiment, located a half-mile underground at the Soudan mine in northern Minnesota, has been searching for WIMPs since 2003. The experiment uses 30 detectors made of crystals of germanium and silicon in an attempt to detect WIMP scatters. The detectors are cooled to temperatures very near absolute zero. Particle interactions in the crystalline detectors deposit energy as heat and as charges that move in an applied electric field. Special sensors detect these signals, which are then amplified and recorded for later study. By comparing the size and relative timing of these two signals, from heat and charge, experimenters can tell whether the particle that interacted in the crystal was a WIMP or a background particle. Layers of shielding materials, as well as the half-mile of rock above the experiment, prevent most of the background particles from reaching the detector.

Previous CDMS data did not yield evidence for WIMPs, but did assure physicists that they had filtered out backgrounds to the level where as few as one WIMP interaction per year could have been detected.

CDMS collaborators are now reporting on their new data set, taken in 2007-2008, which approximately doubles the sum of all past data sets. With each new data set, collaborators must carefully evaluate each detector’s performance, excluding periods when the detectors were not operating properly.

Physicists assess detector operation by frequently exposing the detector to sources of two types of radiation: gamma rays and neutrons. Gamma rays are the principal source of normal matter background in the experiment. Neutrons are the only known type of particle that interacts with germanium nuclei in the same billiard-ball style that WIMPs would. Neutrons frequently hit more than one of the CDMS detectors, while WIMPs would only hit one.

Experimenters use data from these studies as a baseline for determining how well a WIMP-like signal (produced by neutrons) is visible over a background (produced by gamma rays). Based on this information, physicists predict that no more than one background event will be visible in the data region where WIMP signals would appear. Since background and signal regions overlap somewhat, achievement of this background level required experimenters to throw out roughly two-thirds of the data that might contain WIMPs, because these data would contain too many background events.

CDMS experimenters do all of their data analysis without looking at the data region that might contain WIMP events. They use this standard scientific technique, sometimes referred to as “blinding,” to avoid the unintentional bias that might lead them to count background events that have some of the characteristics of WIMP interactions. After collaborators have made detailed estimates of background “leakage” into the WIMP signal region, they “open the box,” or look in that region, to see if they find any WIMP events.

A signal of about five events would meet criteria to claim a dark matter discovery. With only two events detected in this data set, there is about a one in four chance that they could be due to backgrounds. Therefore, CDMS experimenters do not claim to have discovered WIMPs. Previous results have established a rate of interaction between WIMPs and nuclei that varies depending on WIMP mass. The new result improves upon these limits for WIMPs with a large mass. Such upper limits are quite valuable in eliminating a number of theories that might explain dark matter. For example, the results rule out some values that the theory of supersymmetry could have.

What comes next? While physicists could operate the same set of detectors at Soudan for many more years to look for more WIMP events, this would not take advantage of new detector developments and would try the patience of even the most stalwart experimenters (not to mention theorists). A better way to increase sensitivity to WIMPs is to boost the size of detectors that might see the particles, while still maintaining the ability to keep backgrounds under control. This is precisely what CDMS experimenters are now in the process of doing. By summer 2010, collaborators hope to have about three times more germanium nuclei sitting near absolute zero at Soudan, patiently waiting for WIMPs to provide the perfect billiard ball shots that will offer compelling evidence for dark matter.

To that end, the Texas A&M University group, headed by assistant professor of physics and principal investigator Rupak Mahapatra, is currently setting up detector fabrication facilities that will dramatically improve the cost and time it takes to fabricate a detector, through new technology. Discovering the dark matter particle will require significantly larger mass of detectors, and Texas A&M will be playing the central role in the development of the more massive, next-generation SuperCDMS and GEODM experiments.

“It is certainly exciting to have two candidates consistent with nuclear recoil, signature we expect for WIMPs,” Mahapatra said. “However, we need more statistics to be sure. Hence, we need to continue to take data with our new SuperCDMS experiments that are planned to be implemented in 15-kilogram, 100-kilogram and 1,000-kilogram phases. The Texas A&M group is playing a central role in the next-generation SuperCDMS detector fabrication, utilizing more advanced fabrication equipment and newer automated technologies.

“At Texas A&M, we also have experimental high-energy physicist James White, who is working on the competing Liquid Xenon technology that has great potential for discovery of the WIMPs. Additionally, Texas A&M theoretical physicists Richard Arnowitt, Bhaskar Dutta and Dimitri Nanopoulos are well known leaders working in the theoretical aspects of dark matter physics. Thus, it is a very exciting time in WIMP search, in general, and to be at Texas A&M University, in particular.”

For more information on Texas A&M’s involvement in the CDMS experiment and related developments, please see http://faculty.physics.tamu.edu/mahapatra/.

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About CDMS
The CDMS collaboration includes more than 50 scientists from 18 international institutions and receives funding from the U.S. Department of Energy (DOE), the National Science Foundation (NSF), foreign funding agencies in Canada and Switzerland, and from member institutions.

CDMS Participating Institutions
University of California, Berkeley
California Institute of Technology
University of California, Santa Barbara
Case Western Reserve University
University of Colorado-Denver/NIST
Fermi National Accelerator Laboratory
University of Florida
Lawrence Berkeley National Laboratory
Massachusetts Institute of Technology
University of Minnesota
Queen’s University
St. Olaf College
Santa Clara University
Southern Methodist University
Stanford University
Syracuse University
Texas A&M University
University of Zurich

About Fermilab
Fermilab is a DOE Office of Science national laboratory operated under contract by the Fermi Research Alliance, LLC. The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the nation.

About NSF
The National Science Foundation is an independent federal agency that supports fundamental research and education across all fields of science and engineering. NSF funds reach all 50 states through grants to more than 1,700 universities and institutions.

About Research at Texas A&M University
As one of the world’s leading research institutions, Texas A&M University is in the vanguard in making significant contributions to the storehouse of knowledge, including that of science and technology. Research conducted at Texas A&M represents an annual investment of more than $582 million, which ranks third nationally for universities without a medical school, and underwrites approximately 3,500 sponsored projects. That research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world.

Contact: Shana K. Hutchins, (979) 862-1237 or shutchins@science.tamu.edu or Dr. Rupak Mahapatra, (979) 845-8624 or mahapatra@physics.tamu.edu