Scientists may have glimpsed a particle that is a leading candidate for mysterious dark matter but say conclusive evidence remains elusive. A nine-year search from a unique observatory in an old iron mine 2,000 feet underground has yielded two possible detections of weakly interacting massive particles, or WIMPs. But physicists, who include two University of Florida researchers, say there is about a one in four chance that the detections were merely background noise — meaning that a worldwide hunt involving at least two dozen different observatories and hundreds of scientists will continue.
“With one or two events, it’s tough. The numbers are too small,” said Tarek Saab, a UF assistant professor and one of dozens of physicists participating in the Cryogenic Dark Matter Search II, or CDMS II, experiment based in the Soudan mine in Northern Minnesota.
A scientific system buried deep below the Earth, constructed of ultrapure materials held hovering over absolute zero, has finally stirred. This isn’t an attack by misbegotten monsters but an encouraging clue to the main mystery of the universe: dark matter.
The kind of matter with which we are familiar — atoms and molecules, and indeed every particle we have ever created in a laboratory known as baryonic– only makes up about 5% of the universe. Another 25% is dark matter, a kind of particle that is massive and weakly interacting. The remaining 70% is dark energy, which is not even a particle — it’s a smoothly-distributed energy field that remains persistent in density even as the universe expands. The ongoing effort to understand dark matter and dark energy is the most important task of twenty-first century cosmology.
The Cryogenic Dark Matter Search II (CDMS II) does exactly what it says - it’s cryogenically cooled, it’s searching for dark matter, and this is the second time they’ve done it. High purity low temperature crystals of germanium and silicon vibrate are disturbed by anything impacting on them, and they’re buried under seven hundred meters of iron mine to make sure most of “anything” can’t make it. One thing that could conceivably come down and stir things up is a WIMP, a Weakly Interacting Massive Particle - one of the options for dark matter.
The system is so shielded that over an entire year users only expect 0.8 events, and in 2008 they saw two. This is a tantalising taste of data: analysis indicates that the event energy matches the model for dark matter WIMPs, but even after screening out as much noise as possible it simply isn’t enough signal to be sure. Scientists, you see, double-check and confirm things before shouting about them (unlike others who - for example - might hack unprocessed e-mails, strip random sentences out of context, then start screaming about all kinds of nonsense.
This excitement is motivating instead of mob-making: the research team are upgrading the equipment with Super-CDMS stacks of crystal which will triple its efficiency. Space satellites, subterranean sensors, and that little LHC thing: we want this dark matter stuff.
The bulk of the dark matter that makes up 75% of the universe is believed to be nonbaryonic, which means that it contains no atoms and that it does not interact with ordinary matter via electromagnetic forces and includes neutrinos, and possibly hypothetical entities such as axions, or supersymmetric particles. Unlike baryonic dark matter, nonbaryonic dark matter did not contribute to the formation of the elements in the early universe, so its presence is detected only by its gravitational attraction.
Scientists recognized decades ago that the rotational speed of galaxies and the behavior of galaxy clusters could not be explained by the traditional forces of gravity due to the mass of visible stars alone. Something else — something invisible, undetectable yet extremely powerful — had to exert the force required to cause the galaxies’ more-rapid-than-expected rotational speed and similar anomalous observations.
What came to be known as “dark matter” – dark because it neither reflects nor absorbs light in any form, visible or other – is now estimated to comprise as much as 23 percent of the universe. But despite abundant evidence for its influence, no one has ever observed dark matter directly.
There are several possibilities for the composition of this mysterious, omnipresent matter. Particle physics theory points toward WIMPs as one of the most likely candidates.
WIMPs are “weakly interacting” because, although their masses are thought to be comparable to the masses of standard atomic nuclei, they have little or no effect on ordinary matter, and among other things, that makes them extremely difficult to detect. However, scientists believe WIMPs should occasionally “kick” or bounce off standard atomic nuclei, leaving behind a small amount of energy that should be possible to detect.
The CDMS II observatory is located a half-mile underground beneath rock that blocks most particles, such as those accompanying cosmic rays. At the observatory’s heart are 30 hockey-puck-sized germanium and silicon detectors cryogenically frozen to negative 459.58 Fahrenheit, just shy of absolute zero. In theory, WIMPs would be among the few particles that make it all the way through the earth and rock. They would then occasionally kick the atoms on these detectors, generating a tiny amount of heat, a signal that would be observed and recorded on the experiment’s computers.
Observers recorded the two possible WIMP events in 2007, one on Aug. 8 and the second on Oct. 27. Scientists had estimated that five detections would be sufficient to confirm WIMPs — meaning that the two fell short, according to the CDMS. But while the two detections may not be conclusive, they do help to set more stringent values on the WIMPs’ interaction with subatomic particles.
At the very least, the finding helps to eliminate some theories about dark matter — raising the profile of the WIMP and potentially accelerating the race to detect it.
Most experts agree that in the next five years or so, someone will see a clear signal.
Casey Kazan with Luke McKinney via University of Florida
Image at top: Dark Matter ring in Galaxy Cluster CI 0024+17 Hubble Space Telescope
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