Clearest sign yet of dark matter detected
New Scientist Revue, 18 December 2009 by Anil Ananthaswamy
Deep inside an abandoned iron mine in northern Minnesota, physicists may have spotted the clearest signal yet of dark matter, the mysterious stuff that is thought to make up 90 per cent of the mass of the universe.
The Cryogenic Dark Matter Search (CDMS) collaboration has announced that its experiment has seen tantalising glimpses of what could be dark matter.
The CDMS-II experiment operates nearly three-quarters of a kilometre underground in the Soudan mine. It is looking for so-called weakly interacting massive particles (WIMPs), which are thought to make up dark matter.
The experiment consists of five stacks of detectors. Each stack contains six ultra-pure crystals of germanium or silicon at a temperature of 40 millikelvin, a touch above absolute zero. These are designed to detect dark matter particles by looking at the energy released when a particle smashes into a nucleus of germanium or silicon.
The problem is that many other particles – including cosmic rays and those emitted by the radioactivity of surrounding rock – can create signals in the detector that look like dark matter. So the experiment has been carefully designed to shield the crystals from such background “noise”. The idea is that when the detector works for a long time without seeing any background particles, then if it does see something, it’s most likely to be a dark matter particle.
Signal or noise?
When the CDMS-II team looked at the analysis of their latest run – after accounting for all possible background particles and any faulty detectors in their stacks – they were in for a surprise. Their statistical models predicted that they would see 0.8 events during a run between 2007 and 2008, but instead they saw two.
The team is not claiming discovery of dark matter, because the result is not statistically significant. There is a 1-in-4 chance that it is merely due to fluctuations in the background noise. Had the experiment seen five events above the expected background, the claim for having detected dark matter would have been a lot stronger.
Nonetheless, the team cannot dismiss the possibility that the two events are because of dark matter. The two events have characteristics consistent with those expected from WIMPs (PDF).
The CDMS-II team is planning to refine the analysis of their data in the next few months. In addition, they have begun building new detectors in the mine, which will be three times as sensitive as the existing setup. These “SuperCDMS” detectors are expected be in place by middle of next year.
Signs from space
Despite the reservations, there is a palpable sense that an incontrovertible detection of dark matter is imminent. Space-based telescopes like PAMELA have seen particles that could be coming from the annihilation of dark matter in our galaxy. Similar sightings have been made by a balloon-based experiment called ATIC. Soon, the Large Hadron Collider will be starting to smash protons together in the hopes of creating dark matter.
Dan Tovey at the University of Sheffield, UK, who works on the LHC’s ATLAS detector, says that while the CDMS results are not statistically significant, they are bound to generate excitement at the LHC. “I’m sure that people will be looking at [these results] with a lot of interest,” he says.
He points out that even if direct detection experiments like CDMS find evidence of dark matter, the LHC will have to create them in order for us to understand the underlying physics. For instance, the theory of supersymmetry predicts a kind of dark matter that will be the target of searches at the LHC.
“The really exciting aspect of all this is that if you see a signal in a direct-detection dark matter experiment and a signal for supersymmetry at the LHC, you can compare those two observations and investigate whether they are compatible with each other,” says Tovey.
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