Elusive dark matter found at last?

Scientists may have finally detected concrete signs of the long-sought-after "dark matter" —thought to make up at least one-fourth of the universe— after an international collaboration claimed to observe a weakly interacting massive particle (WIMP)-like signal.

 

Researchers at the international Super Cryogenic Dark Matter Search (SuperCDMS) experiment described the WIMP-like signal as at the 3-sigma level, or a 99.8-percent chance.

 

But Texas A&M high-energy physicist Rupak Mahapatra said that while this finding is "certainly very exciting," they need more data to back it up.

 

Mahapatra is a principal investigator in the collaboration and a member of the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy.

 

"In high-energy physics, a discovery is only claimed at 5-sigma or better. 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," he said (http://www.science.tamu.edu/articles/1052).

 

"We are only 99.8 percent sure, and we want to be 99.9999 percent sure. At 3-sigma, you have a hint of something. At 4-sigma, you have evidence. At 5-sigma, you have a discovery," he added.

 

Besides, he said the $30 million involved in this research means their work "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," he said.

 

Importance of dark matter

 

Dark matter is believed to make up nearly a quarter of the universe, holding the cosmos together. To date however, it has yet to be directly observed.

 

The CDMS experiment is 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.

 

CDMS uses sophisticated detector technology and advanced analysis techniques to let cryogenically cooled (almost absolute zero temperature) germanium and silicon targets to search for the rare recoil of dark matter particles.

 

Elusive WIMPs

 

WIMPs are elusive and rarely interact with normal matter. Scientists believe they occasionally bounce off or scatter from atomic nuclei.

 

In doing so, they leave a small amount of energy that can be 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) on the International Space Station (ISS).

 

Mahapatra said the latest analysis stemmed from comprehensive data taken from the largest exposure with silicon detectors during the CDMS-II operation.

 

CDMS-II was 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 equals a billion electron volts] but more sensitive for lower masses. The analysis resulted in three events, and the estimated background is 0.7 events," he added.

 

Mahapatra also said the Texas A&M group performed the crucial calibration of the silicon detectors, such that the signal would look the same, regardless in which of the eight detectors located within the mine it might appear.

 

While Mahapatra said Monte Carlo simulations could not rule out statistical fluctuations as the cause of the backgrounds, the fluctuations would rarely produce a similar energy distribution, which may be interpreted as spin-independent scattering of WIMPs.

 

In 2010, the collaboration used germanium detectors and reported the detection of two events in the signal region and an estimated background of 0.9 events.

 

But they concluded these events more likely were attributable to leakage of surface electrons rather than actual nuclear recoils.

 

Since 2009, Mahapatra and his Texas A&M team, including 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 for the project's current phases.

 

These include SuperCDMS to the even more sophisticated GEODM (Germanium Observatory for Dark Matter) experiments.

 

He said his laboratory's new six-inch diameter silicon detectors represent a world-first in cryogenic detection and are about 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. We also have a 90 percent success rate, versus the previous 20 percent rate for the original silicon and germanium devices," Mahapatra said.

 

Continuing probe

 

Mahapatra said the collaboration will continue to probe this WIMP sector using the SuperCDMS Soudan experiment's operating germanium detectors.

 

It is also considering using silicon detectors in future experiments. — TJD, GMA News