Scientists see promising blips in underground dark matter search

Three potential signatures of exotic dark matter particles have been found hidden in the readings from an underground lab in Minnesota  — and although the results are too tentative to be classified as a discovery, scientists say they provide promising new clues to the solution of a decades-old mystery.

"People shouldn't come away from this thinking that we've found dark matter," Rupak Mahapatra, a physicist at Texas A&M University who is a principal investigator with the SuperCDMS collaboration, told NBC News. "Really, it's just the beginning. ... What we really need to do is make more detectors and run them, and be sure."

If the results are confirmed, that would point to the existence of a weakly interacting massive particle, or WIMP, that could help account for the 27 percent of the universe that is thought to consist of dark matter. Such matter seems to be invisible and is detected primarily through its gravitational effect. Another mysterious quality known as dark energy accounts for 68 percent of the universe. That leaves just 5 percent consisting of ordinary matter — the stuff that makes up everything we see around us.

Physicists have puzzled over the nature of dark matter since the 1930s, and billions of dollars have been spent building experiments to track it down. 

Finding, or fluke?

The three high-energy events were recorded in 2008 by the Cryogenic Dark Matter Search, or CDMS, an experiment that was set up a decade ago nearly a half-mile (713 meters) underground in northern Minnesota's Soudan mine. That depth helps to shield the experiment from background cosmic rays that would overwhelm the signature of dark matter interactions at the surface.

The interactions seen by the CDMS team point to the existence of WIMPs with a best-guess mass of 8.6 billion electron volts, which would be about nine times as massive as the proton. Scientists calculated that there should be, on average, 0.7 events of that type recorded during the time frame for the readings.

It's possible that the three events are statistical flukes — analogous to, say, rolling three 7's in a row at a Vegas craps table. In this case, the scientists say there's a 99.8 percent chance that their results reflect a real phenomenon rather than a random crap shoot. That's significant, but it's not significant enough to claim a discovery. To make such a claim, the confidence level would have to go up to 99.9999 percent, or 5-sigma in math-geek speak.

"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," Mahapatra explained in a Texas A&M news release. "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."

Mahapatra said it took almost five years to notice the potential dark matter events because the CDMS team began their analysis by looking at the results from a set of germanium detectors, which are sensitive to higher masses. Another set of data was collected using silicon detectors, which are sensitive to lower masses, but those readings were put aside.

In the past few years, other dark-matter experiments began pointing to a mass range that was lower than scientists expected. "When they started seeing something significant, we thought we would look at our silicon data, which we were sitting on for more than four years," Mahapatra said.

'Hot on the trail'

Caltech theoretical physicist Sean M. Carroll agreed that it was too early to declare a discovery, but said "it would not be a surprise" if the CDMS data ended up being confirmed. Other experiments, ranging from AMS to LUX to SuperCDMS to Xenon1T, will be adding to the evidence. "It is certainly a reminder that we are hot on the trail of looking for dark matter," Carroll told NBC News.

He was intrigued by the possibility that the heavier particle mass could explain why dark matter accounts for so much more of the universe than ordinary matter. "You can imagine that there is one dark matter particle for every ordinary particle," Carroll said.

Some theorists propose that ordinary matter and dark matter come into existence through a process known as cosmic cladogenesis. Mahapatra said a balance in the number of the two types of particles would fit such a hypothesis. "It's either a coincidence, or a tremendous clue," Mahapatra said.

 

Editor's Note: The Cynthia and George Mitchell Foundation provided start up funding and continues to provide significant research and program gifts for the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University.

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