A race is on in Case Western Reserve University's physics department and around the world to be the first research group to capture signals from WIMPs (weakly interactive massive particles)—the substance that comprises dark matter.
Experts in the field suspect whoever tracks and verifies WIMP signals first will be the leading contender for a Nobel Prize in physics. Two of the top competitors currently work just down the hall from each other in the physics department at Case Western Reserve: Thomas Shutt and Daniel Akerib.
"It is exciting to have key players from the two most powerful dark matter detection searches in the world here in our department," said Lawrence M. Krauss, the Ambrose Swasey Professor of Physics and Astronomy at Case Western Reserve. "If dark matter particles are directly detected one day, it is very likely to be with one of these experiments, and we will be proud of the remarkable efforts of both groups even as we bask in the realization that either way, the Case Western Reserve physics department has taken the lead in this most important endeavor."
"WIMPS have become a hot topic worldwide," said Shutt, the Agnar Pytte Professor of Physics at Case Western Reserve. But he cautions this search is no easy task and likens it to trying to find a needle in a haystack.
Scientists around the world hypothesize that millions of WIMPs continually pass through the Earth every second, but have eluded their detection. The presence of WIMPs is suspected because the rate of the expansion of the universe is slowed by a gravitational pull from some mass that cannot be explained by the amount of planetary and other matter in the universe.
Shutt is a lead spokesperson, along with Richard Gaiskell from Brown University, for LUX (the Large Underground Xenon dark matter experiment), a 10-institution collaboration. The researchers are utilizing the abandoned Homestake gold mine in Lead, S.D., as a site to set up a new WIMP detector some 4,850 feet below the Earth's surface.
Going underground helps filter out some of the cosmic radioactive gamma, beta and x-ray particles that naturally occur in the environment, resulting in about 10,000 hits a second to the Earth's surface. Deep in the mine, and with the use of xenon, a noble gas that naturally blocks the passage of most radioactive particles, the number of hits can be reduced to approximately three per year—compared to the 100 billion or more annually at the surface.
Shutt's top competitor, Akerib, chair and professor of physics at Case Western Reserve, is just doors down in the department offices in Rockefeller Hall on campus. He is a member of the Cryogenic Dark Matter Search project.
Akerib's search takes place approximately 2,341 feet underground at the former Soudan iron mine in Minnesota, and like Homestake, uses the Earth to filter cosmic and radioactive particles. Akerib's research group at Case Western Reserve helped build and operate the detector for this site.
According to Akerib, Case Western Reserve has a major role in two of only three underground projects in the search for dark matter operating in the United States. Similar experiments also are taking place in Europe, Japan and Canada.
"While these experiments are competitive, they are also complementary in many ways," Krauss said. "It is important, when trying to address one of the most important, outstanding puzzles in cosmology, to attack the problem from many directions," Krauss continued. "Dan and Tom, and their groups, actually benefit from the competition and also from the many collaborative discussions that occur in the department."
Though Akerib and Shutt lead competing projects, they soon will be sharing some research resources in addition to taking part in joint discussions.
The Homestake site where Shutt works is the targeted home for the National Science Foundation's new Deep Underground Science and Engineering Laboratory (DUSEL), a proposed new national laboratory for underground science. Akerib serves as co-chair of the dark matter working group for the DUSEL proposal process.
Before the national DUSEL laboratory even begins construction as early as 2011-12, the State of South Dakota and credit card magnate T. Denny Sanford have jump started the process by using $100 million to refurbish the Homestake mine as an interim laboratory called the Sanford Undgerground Science and Engineering Laboratory, or SUSEL.
With Shutt's guidance, LUX will be the flagship first experiment at SUSEL, and will be proposing a larger experiment for the time when DUSEL is funded. Eventually, when DUSEL is fully operational, the Homestake site will have facilities that reach a depth of 7,800 feet and cost about $500 million for infrastructure and research.
Shutt's research group at Case has had a major role in designing, testing and integrating a new detector for Homestake.
The detector utilizes xenon, one of the noble gases like helium, argon and krypton. Xenon is the heaviest of the noble gases. When xenon is cooled to -100 degrees Celsius, it becomes a liquid. The detector uses both the large pool of liquefied xenon, as well as a layer of gaseous xenon above it.
As WIMPs strike the xenon atoms, a flash of light is emitted and recorded with photo sensors. Electrons come off the atom at the time of the impact and will be pulled through an electrical field up out of the liquid, where they will again emit a light flash when encountering xenon gas atoms. The principle behind the detector is to find a ratio between light emitted upon impact and the ionization rate during the impact in the xenon gas. This ratio should provide a unique signal for WIMPS.
For the stray radioactive or cosmic ray particles that make it into the detector, the researchers should be able to distinguish a WIMP from those particles by the number of times electrons are scattered in the liquid xenon.
WIMPS differ from other radioactive particles in that they hit the nucleus of the atom instead of the electrons on the nucleus' surface, giving a different signal in the way the atom moves or recoils at impact.
Shutt was involved in prior dark matter research for the Xenon10project that operated in what Shutt describes as a "James Bond-like" underground laboratory in Italy where three underground halls the size of football fields are filled with a variety of dark matter and neutrino experiments. That project is the precursor to the Homestake experiment that is expected to begin in late summer, said Shutt.
Akerib and Shutt are not the only Case Western Reserve physicists to use mines to find cosmic particles. The late Frederick Reines, a former faculty member in physics, shared the Nobel Prize for his discovery of neutrinos with Clyde Cowan. They detected them through an experiment operating in an area salt mine.
Case Western Reserve currently is one of 10 institutions in the LUX collaboration to search for WIMPS and is joined by Brown University, Lawrence Livermore National Laboratory, Lawrence Berkeley National Laboratory at Berkeley, Maryland University, University of California at Davis, University of California at Los Angeles, the University of Rochester and Yale University in the dark matter search.
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