The shaft leading to the laboratory a half-mile underground.
By Sharon Schmickle | Friday, Dec. 18, 2009
After six years of chasing elusive dark matter in the depths of northern Minnesota's old Soudan mine, researchers report that they have detected two signals from what could be the mysterious particles believed to function as invisible glue that binds the universe.
If, indeed, the particles are dark matter, this would be a colossal finding, leading to a better understanding of everything from the origins of the universe to the forces that surround us here on Earth today.
Scientists from the University of Minnesota and 17 other institutions collaborating on the quest for dark matter were cautious, though, in explaining their findings on Thursday: Two times, their experiments have detected signals from particles with characteristics "completely consistent" with those expected from dark matter, said a statement issued by the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois.
But there is a one-in-four chance the signals came from far more ordinary stuff — the background chatter of radioactive decays and cosmic rays.
Under the strictest scientific criteria, those odds aren't good enough to pop the champagne corks just yet. They leave some doubt that dark matter's Weakly Interacting Massive Particles (WIMPs) truly have been detected.
"So we can make no claim to have discovered WIMPS," the research team said.
Still, the news was hailed worldwide.
"The claim, if confirmed next year, will rank as one the most spectacular discoveries in physics in the past century," said the Guardian of London.
"Though tentative, tonight's results triggered an immediate wave of excitement in the science community," the Guardian said.
"The stakes for astronomy and physics could hardly be greater," the New York Times said. "If the particles are confirmed by tests at other detectors, it would mean that, after more than half a century of speculation, astronomers are zeroing in on the identity of the invisible material that . . . determines the architecture of the visible universe."
Rumors . . . expectations . . . reality
In anticipation of the news, rumors had circulated for days on physicists' blogs that a major breakthrough was imminent.
Skeptics in that rumor churn downplayed in advance the significance of any discovery.
They had good reason for doubt — even before they knew for sure what was coming. A research team in Italy reported detecting interactions of dark matter particles in 2000, but other experiments have not confirmed this result. Another study conducted underground at Stanford University in California found that what had appeared to be dark-matter effects were coming from other, less interesting particles.
Expectations mounted nevertheless.
Workers take steps down the shaft leading to the underground laboratory.
A blogger for Discover Magazine wrote just before the news was released: "The excitement in the air is palpable. Not much work is being done — everyone is pretty much talking in the hallways."
Scientists at the U of M who played a role in the Soudan experiment have been tight-lipped about their findings pending upcoming publication in a scientific journal.
To put rumors to rest, though, research team leaders did provide briefings on the findings late Thursday at Fermilab in Illinois and the SLAC National Accelerator Laboratory in Menlo Park, Calif.
At the U of M today, Oleg Kamaev, a postdoctoral researcher involved with the project will present a special seminar on the experiment in Room 435 Tate Laboratory of Physics, 116 Church St. SE., Minneapolis, at 2:30 p.m.
Two U of M physics professors who were prominent in the research are Priscilla Cushman and Vuk Mandic. Several graduate students and others were involved too.
From Einstein to corn rows
Despite the excitement, no one is absolutely sure WIMPS exist. And no one knows at this point whether any practical use could be made of their discovery other than vastly enhancing understanding of nature.
That begs the question of why scientists would throw so much effort into detecting them.
U of M physics professor Marvin Marshak explained it this way: "When Albert Einstein published theory of general relativity in 1915, we could hardly imagine it would be used today for GPS satellites that would help farmers plant straight rows of corn. . . . A lot of research one does to answer these fundamental questions ends up having no practical applications. But the few gems in there end up having applications that are so huge they change the way that people live on Earth."
Slipping through your body
All of this intrigue is over mysterious particles that, in theory, continually slip through everything we can see — including your body while you read this report.
They are invisible to us because they don't emit or absorb light. They are neutral, lacking an electrical charge which would have made them easy to detect. And they rarely engage with other matter, so they can pass stealthily through the atoms that comprise your body and everything else around you. That's why scientists call one class of these theoretical particles "weakly-interacting," or WIMPs.
So why think they are there at all? The answer is an unsolved puzzle of gravity.
Laws of gravity as we know it don't explain the motions of the galaxies unless some invisible matter is factored into the equations. The violent force of the Big Bang set off an expansion so rapid that it should have ripped the universe apart. Instead, planets and galaxies formed and moved in the orderly fashion we expect to see when we watch the night sky.
A scientist observes detectors set up to interact with particles from dark matter.
Based on astronomical observations from telescopes, x-ray satellites and cosmic microwave background measurements, scientists believe dark matter, consisting of tiny particles produced in the Big Bang, would have provided the gravitational tugs that caused matter to coalesce into galaxies.
Further, they think our own Milky Way galaxy is embedded within an enormous ball of dark matter. And our solar system rotates through this cloud of matter.
Dark matter not only surrounds us, but it outweighs everything we can see by a very large factor.
What's perplexing is that scientists know very little about the nature of that ubiquitous other matter.
High-stakes billiards
The collaboration working with the U of M to unlock that mystery is called the Cryogenic Dark Matter Search (CDMS) experiment. Their $15 million study is funded by the U.S. Department of Energy and the National Science Foundation.
Their laboratory is under 2,000 feet of rock at the former Soudan mine on Minnesota's Iron Range.
There, 30 super-chilled detectors the size and shape of hockey pucks are set up to catch measures of the subtle effects that could be expected if they are bumped by a WIMP.
Think of a billiard ball being struck by the cue ball. That's the dynamic for one measure the researchers are seeking. The idea is that WIMPS should occasionally collide with crystals of the semiconductors germanium and silicon at the core of the detectors and recoil, giving off a miniscule amount of energy in the form of heat and electrical charge. Sophisticated sensors detect those recoils, amplify their signals and record them in computers for analysis.
A huge part of the challenge is filtering out every other possible explanation for that energy — from a dust particle to a cosmic ray.
In an initial testing round between 2006 and 2007, the researchers reported finding nothing. But they said those tests helped refine the understanding and suppression of background interference. Among other steps, they were able to evaluate the performance of each detector and exclude periods when they weren't operating properly.
Since then, they have taken steps to double the sensitivity of the tests.
Now they are reporting analysis of readings taken between 2007 and 2008.
The race to confirm begins now
While the findings do not show conclusively that they hit a bulls eye during that period, they do enhance anticipation over the next steps.
Plans call for ramping up the detection capacity at the Soudan site.
"By summer of 2010, we hope to have about three times more Germanium nuclei sitting near absolute zero at Soudan, patiently waiting for WIMPs to come along and provide the perfect billiard ball shots that will offer compelling evidence for the direct detection of dark matter in the laboratory," said the Fermilab statement.
Spurred by this news, other labs around the world will race to try to confirm the findings or directly prove dark matter in their own way. The XENON100 dark matter search experiment already was rivaling the Soudan team. It is operated in Italy by Columbia and Rice universities and the University of California in Los Angeles with funding from the U.S. National Science Foundation and European agencies.
Taking a different approach, scientists operating the world's largest particle accelerator — the Large Hadron Collider in Geneva, Switzerland — have geared up for experiments that also could reveal the nature of dark matter.
