Dark Matter Search Results Indicate First Hint of WIMP-like Signal
COLLEGE STATION -- An international collaboration whose search for dark matter is powered by detectors being fabricated at Texas A&M University has for the first time observed a concrete hint of what physicists believe to be the particle behind dark matter and therefore nearly a quarter of the universe -- a WIMP, or weakly interacting massive particle.
Scientists with the international Super Cryogenic Dark Matter Search (SuperCDMS) experiment involving Texas A&M high-energy physicist Rupak Mahapatra are reporting a WIMP-like signal at the 3-sigma level, indicating a 99.8 percent chance -- or, in high-energy parlance, a hint of the mysterious substance dark matter that is believed to hold the cosmos together but to date has never been directly observed.
"In high-energy physics, a discovery is only claimed at 5-sigma or better," Mahapatra said. "So this is certainly very exciting, but not fully convincing by the standards. We just need more data to be sure. For now, we have to live with this tantalizing hint of one of the biggest puzzles of our time."
SuperCDMS researchers are announcing their breakthrough result in talks around the nation, including one at noon today (Monday, April 15) by Mahapatra, a principal investigator in the collaboration and a member of the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy. Mahapatra's public presentation will be held in the Stephen W. Hawking Auditorium within the Mitchell Institute and streamed live via TTVN. The collaboration has detailed its full results in a paper published in arXiv that eventually will appear in Physical Review Letters.
Notoriously elusive, WIMPs rarely interact with normal matter and therefore are difficult to detect. Scientists believe they occasionally bounce off, or scatter like billiard balls from, atomic nuclei, leaving behind a small amount of energy capable of being tracked by detectors deep underground, particle colliders such as the Large Hadron Collider at CERN and even instruments in space like the Alpha Magnetic Spectrometer (AMS) mounted on the International Space Station (ISS).
The CDMS experiment, located a half-mile underground at the Soudan mine in northern Minnesota and managed by the United States Department of Energy's Fermi National Accelerator Laboratory, has been searching for dark matter since 2003. The experiment uses very sophisticated detector technology and advanced analysis techniques to enable cryogenically cooled (almost absolute zero temperature at -460 degrees F) germanium and silicon targets to search for the rare recoil of dark matter particles.
Mahapatra says the latest analysis represents comprehensive data gleaned from the largest exposure with silicon detectors during the CDMS-II operation, an earlier phase of the overall experiment involving more than 50 scientists from 18 international institutions.
"This result is from data taken a few years ago using silicon detectors manufactured at Stanford that are now defunct," Mahapatra said. "Increased interest in the low mass WIMP region motivated us to complete the analysis of the silicon-detector exposure, which is less sensitive than germanium for WIMP masses above 15 giga-electronvolts [one GeVa is equal to a billion electron volts] but more sensitive for lower masses. The analysis resulted in three events, and the estimated background is 0.7 events."
In addition to being heavily involved in the data analysis, Mahapatra says the Texas A&M group performed the crucial calibration of the silicon detectors, guaranteeing that the signal would look the same, regardless in which of the eight detectors located within the mine it might appear.
While Mahapatra says Monte Carlo simulations weren't able to rule out statistical fluctuations as the cause of the backgrounds, the team believes said fluctuations would rarely produce a similar energy distribution, which they interpret instead as spin-independent scattering of WIMPs. And although he says the result is certainly encouraging and worthy of further investigation, he cautions it should not be considered a discovery.
"We are only 99.8 percent sure, and we want to be 99.9999 percent sure," Mahapatra said. "At 3-sigma, you have a hint of something. At 4-sigma, you have evidence. At 5-sigma, you have a discovery.
"In medicine, you can say you are curing 99.8 percent of the cases, and that's OK. When you say you've made a fundamental discovery in high-energy physics, you can't be wrong. Given the money involved -- $30 million in this case -- it has to be extremely precise. With a 99.8 percent chance, that means if you repeated the same experiment a few hundred times, there is one chance it can go wrong. We want one out of a million instead."
Using germanium detectors, the collaboration previously in 2010 reported detection of two events in the signal region and an estimated background of 0.9 events. They eventually concluded these events more likely were attributable to leakage of surface electrons rather than actual nuclear recoils.
For the past four years, Mahapatra and his Texas A&M team -- which includes his Department of Physics and Astronomy-based research group as well as collaborator Rusty Harris in the Department of Electrical Engineering -- have been developing the larger, more advanced detectors needed for the project's current phases, from SuperCDMS to the even more sophisticated GEODM (Germanium Observatory for Dark Matter) experiments. They are developing both germanium and silicon detectors to create dual-threat devices that are much bigger, better and cheaper. He notes his laboratory's new 6-inch diameter silicon detectors represent a world-first in cryogenic detection and are approximately 30 times more sensitive than the individual silicon detectors behind this latest result.
"The industrial manufacturing and fabrication facility we have set up here at Texas A&M has enabled us to bring down the cost from $350,000 per kilogram to about $40,000 per kilogram," Mahapatra said. "We also have a 90 percent success rate, versus the previous 20 percent rate for the original silicon and germanium devices."
Mahapatra says the collaboration will continue to probe this WIMP sector using the SuperCDMS Soudan experiment's operating germanium detectors and is considering using silicon detectors in future experiments.
The collaboration's work -- beginning with CDMS and CDMS-II and continuing with SuperCDMS and GEODM -- is funded by the DOE and the National Science Foundation as well as the Natural Sciences and Engineering Research Council of Canada (NSERC).
"In addition to NSF funding and an early career research award from the DOE, this work would not have been possible without start-up support from Texas A&M University and the College of Science and roughly $2 million in equipment from Maximum Integrated Products, Inc. (NASDAQ: MXIM) in Dallas," Mahapatra said. "Additionally, the Mitchell Institute funds the postdoctoral fellow, Joel Sander, who is spearheading the effort to develop alternate next-generation detectors that are not only another order of magnitude cheaper and but can also run at easier cryogenic temperature, which is ideal for a ton-scale experiment on budget. The goal of this funding was to take up high-risk, high-return research that normally does not get supported by traditional funding from federal funding agencies like DOE and NSF."
For more information on the Super Cryogenic Dark Matter Search Experiment, go to http://cdms.berkeley.edu/.
To learn more about Mahapatra's involvement with the collaboration and related research, visit http://faculty.physics.tamu.edu/mahapatra/.
Click here for a 2009 podcast on WIMPs by the 365 Days of Astronomy project.
Finally, here is a brief overview on dark matter from Ph.D. Comics.
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Contact: Shana K. Hutchins, (979) 862-1237 or firstname.lastname@example.org or Dr. Rupak Mahapatra, (979) 229-4196 or email@example.com