The evidence for dark matter is comprehensive. We've measured its effect on galaxies and galaxy clusters, and we've seen its imprint in gravitational lenses and the cosmic microwave background. The annoying thing is that we still don't know what it is. All the evidence indicates that dark matter is likely to be a weakly interacting massive particle (or WIMP), but the best we've found when searching for this particle is a few intriguing hints of evidence.
In the latest hint, the people behind a detector that sits deep in a Minnesota mine say they've observed three events that appear likely to be the faint impact of dark matter particles. Unfortunately, by every measure they try, the significance of these events falls short of discovery. We're therefore left waiting for better detectors and more data—just as we were the last time this team announced a similar result.
Since the evidence indicates that dark matter is a particle (after all, it had to be around before there were even atoms in order to influence the cosmic microwave background), researchers have focused on three ways we might detect a particle that doesn't fit into the existing Standard Model. One track involves searches through the data in the LHC, looking for signs that some of the mass produced in a collision is being carried away by something we can't detect. So far, that hunt has come up blank.
Another option is to look for evidence of dark matter particles colliding with each other. There are a lot of these out in the Universe, and when they smack into each other, they should produce some indication of it. But those results are, at best, ambiguous. They certainly don't provide any indication that we're on the verge of definitive evidence for these particles.
So we're left with the third option: direct detection. Although dark matter is usually presented as avoiding any interactions with regular matter, that's not entirely true under every model. In some models, there's a tiny potential for dark matter to interact with the sort of matter we're all familiar with (in technical terms, this is called a cross section). Because it's so small, however, even a massive dark matter particle—remember, classified as a WIMP—that smashes into an atom will only end up giving it a gentle nudge. This makes detecting these interactions very difficult, since so little energy is involved.
Nevertheless, the Cryogenic Dark Matter Search (CDMS) has been trying to detect this faint hint of a nudge for a number of years. To get rid of many sources of background, the detector was buried in a mine in Minnesota. To get the atoms of its detectors (it uses both silicon and germanium detectors) to settle down enough so that the impact of a WIMP would be apparent above the background of normal movement, the detectors were chilled to 40 milliKelvin. And then, with everything in place, the researchers waited, gathering data throughout five years.
In the experiments the team is now announcing, the researchers took data from four runs of the silicon detectors, performed between 2007 and 2008, after they worked a few bugs out of the system in the earlier runs. During that time, most of the events the researchers were able to detect came from electrons bumping around in the detector, but these could be filtered out because they're usually associated with some change in charge. They also detected the decay of some radioactive elements in the mine itself. When all that was eliminated, the remaining sources of background were expected to be stray neutrons produced by cosmic rays, but the detector was unlikely to have seen any of these in the time that it ran (the actual calculations suggest it would see 0.13 such events).
With all that accounted for, there were still three events that stood out from the background. Statistically modeling these results, the researchers found that the results favor a WiMP-and-background explanation for the data over a background-only explanation, but the deviation from background-only hasn't reached the five-sigma standard for discovery. Intriguing, but there is still more work to do.
That explanation isn't the only thing the resarchers can tell about the data. Each of these events deposited on the order of 10 kilo-electron Volts into the detector. Working backward, the team calculates that this implies the WIMP they're detecting has a mass of 8.6GeV. That number is informative for a number of reasons. One is that previous direct detection experiments (primarily DAMA/LIBRA and CoGent) have also found hints of a signal at similar energies. These hints may sound promising, but another experiment based on liquid xenon detectors excluded this mass, and theoretical considerations had suggested that any WIMPS should be heavier.
For the second time this month, we're left with a tantalizing hint of a signal in the search for dark matter. Ultimately, that's something well short of satisfying.