Researchers in northern Minn. seek to identify dark matter

Data from the Soudan Mine are just one small piece of the puzzle.

Brent Renneke

Data released in December from the Soudan Underground Mine in northern Minnesota have brought scientists one step closer to identifying dark matter, which makes up 25 percent of the universeâÄôs unknowns. With EarthâÄôs known matter composing only 5 percent of the universe, the remaining 95 percent is made up of basic unknowns âÄî one being a force called dark energy, and the other being dark matter. Today, the mine, along with other locations around the world, is working to develop methods to expand on the December findings. Researchers from the University of Minnesota and 13 other institutions worked on the Cryogenic Dark Matter Search (CDMS II) experiment, which placed 30 detectors almost a half-mile underground to detect weakly interacting mass particles that may compose dark matter. It recorded two occurrences with a 75 percent certainty that it is an unidentified particle, which may be dark matter. Priscilla Cushman, a physics professor at the University and a researcher at the mine, said the findings are far from identifying dark matter. She said there was a 75 percent chance the occurrences are an unknown particle and a 25 percent chance the occurrences were simply background noise. âÄúThat would assume the only other unknown particle that exists is dark matter,âÄù Cushman said. In the near future, the goal for many institutions is to reduce the current uncertainty about dark matter. Xenon 100 Xenon 100 is a project also dedicated to the search for dark matter. With about a dozen institutions involved, the detector is based out of Gran Sasso National Laboratory in Italy. Using fluid xenon to detect the weakly interacting mass particles, the Xenon 100 detects at a level of sensitivity that is considerably higher than the CDMS II experiment, according to Uwe Oberlack, professor of physics and astronomy at Rice University. Uberlack said the goal of the Xenon 100 is to be 20 times more sensitive than the CDMS II experiment. However, Uberlack said this will not be achieved on day one. âÄúThat is not going to be the first result, because you have to accumulate extra exposure to achieve those levels,âÄù Oberlack said. What the Xenon 100 does have a chance to achieve with its first set of data released this summer is to prove or disprove CDMS IIâÄôs December data, according to Vuk Mandic, assistant professor of physics at the University and one of the researchers involved in the CDMS II experiment. âÄúThey may either confirm our findings, in which case it would be very interesting,âÄù Mandic said, âÄúor they will show that what we are seeing is some misidentified background event.âÄù Mandic said the results of the Xenon 100 should be comparable to or greater than the CDMS II results. Regardless of the data the Xenon 100 delivers this summer, Oberlack said he fully expects a second run to follow this summerâÄôs data. SuperCDMS Researchers at the Soudan Mine are in the process of replacing the detectors from the CDMS II experiment. The project is called SuperCDMS, which will lead to greater sensitivity to any possible weakly interacting mass particle detection. Oberlack said the CDMS II experiment reached the end of the line in terms of being able to improve the detector. âÄúIn a way, you just cannot make it any more sensitive,âÄù he said. CDMS II used four kilograms of germanium to detect the vibrations of particles. The SuperCDMS will also use the chemical element, but it will increase the amount to 15 kilograms, according to Stanford University physics professor Dr. Blas Cabrera. Cabrera said this, along with the new experiment running for two years, accounts for the improvement. âÄúThe increase of germanium and the length of time will give us a 10-times increase in sensitivity,âÄù he said. The approval for construction of SuperCDMS was not related to CDMS IIâÄôs findings, according to Cabrera. âÄúIndependent of exactly what CDMS II reported, it would make sense to continue the experiment with something that is 10 times more sensitive,âÄù he said. The timeline for the SuperCDMS is still undetermined because the group has not decided how much data they want to acquire before they analyze, Cabrera said. âÄúThere may be data presented at the end of 2011 or the end of 2012 depending on how we decide we would like to do it,âÄù he said. Required collaboration If the presence of dark matter is to become certain, both Cabrera and Oberlack said a variety of experiments required to prove its existence. âÄúThe feeling we have is that it is important in this field to have multiple technologies,âÄù Cabrera said. âÄúWe want high-caliber going on with high-caliber, and the Xenon 100 is in that class.âÄù Oberlack said that the presence of another method is required despite their findings. âÄúYou really want to have more than one experiment running, because if only we find something, no one in this industry is going to believe we have dark matter,âÄù he said.