April 16, 2013
WIMP Experiment Points To First Concrete Evidence Of Dark Matter
April Flowers for redOrbit.com - Your Universe Online
Using detectors created at Stanford, an international collaboration of scientists has, for the first time, observed a concrete hint of a WIMP — weakly interacting massive particle. Physicists believe the WIMPs could be the particles behind the mysterious phenomenon of dark matter, which is thought to make up nearly a quarter of the Universe´s mass-energy.
Texas A&M high-energy physicist Rupak Mahaptra is part of the Super Cryogenic Dark Matter Search (SuperCDMS) experiment which has reported a WIMP-like signal at the 3-sigma level, indicating a 99.8 percent chance of the mysterious substance dark matter that is believed to hold the cosmos together. To date, however, it 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."
The findings of this breakthrough study have been announced in a series of talks taking place around the US, and detailed in a paper published online in arXiv. It will eventually be published in the journal Physical Review Letters.
WIMP's are notoriously elusive and rarely interact with normal matter, making them very difficult to detect. They are thought to occasionally bounce off of, or scatter like billiard balls struck by the cue, atomic nuclei. This leaves behind small amounts of energy capable of being tracked by particle colliders, like the Large Hadron Collider (LHC) at CERN, buried deep underground, or even by the Alpha Magnetic Spectrometer (AMS) mounted on the International Space Station (ISS).
Located a half mile underground in the Soudan mine of northern Minnesota, the CDMS experiment is managed by the US Department of Energy's Fermi National Accelerator Laboratory. CDMS has been searching for dark matter since 2003, using very sophisticated detector technology and advanced analysis techniques. These enable cryogenically cooled germanium and silicon targets to search for the rare recoil of dark matter particles at temperatures approaching absolute zero (-460 degrees F).
According to Mahaptra, the latest analysis represents comprehensive data compiled from the largest exposure with silicon detectors during the CDMS-II operation, an earlier phase of the overall experiment. The total CDMS collaboration consists of 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."
Mahaptra said the Texas A&M group not only was heavily involved in the data analysis but also performed the crucial calibration of the silicon detectors, which guaranteed the signal would look the same, regardless of which of the eight detectors within the mine it might appear within.
The Monte Carlo simulations were not able to rule out statistical fluctuations as the cause of the backgrounds, according to Mahaptra. However, the team believes such fluctuations would rarely produce a similar energy distribution. They interpret this distribution as spin-independent scattering of WIMPs. Mahaptra cautions that although the result is encouraging and worth further study, 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."