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WIMP-like signal indicates first hint of mysterious dark matter

Researchers have 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.

Washington: An international team of researchers 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 `n` 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. 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," Mahapatra said.

Mahapatra said 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.

In addition to being heavily involved in the data analysis, Mahapatra said the Texas A `n` 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 said 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 said the result is certainly encouraging and worthy of further investigation, he cautions it should not be considered a discovery.

For the past four years, Mahapatra and his Texas A `n` 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 `n` M has enabled us to bring down the cost from 350,000 dollars per kilogram to about 40,000 dollars 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.

Mahapatra said 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 has detailed its full results in a paper published in arXiv that eventually will appear in Physical Review Letters.


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