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A Hint of Dark Matter Lies Deep Underground in Minnesota

Make some popcorn, physics-spotters. We're just getting started.
April 17, 2013, 2:30pm

If you thought discovering (or likely discovering) the Higgs boson was a long slog of rumors, data sifting, "hints," and more waiting, the hunt for dark matter is likely to send you over the edge. (See: "What the Fuck Is Dark Matter?") With the Higgs we at least had just one big super-expensive particle smashing machine to lavish our attentions on and just one proposed particle with a set of somewhat known parameters.


With the dark stuff, we have multiple methods of detection taking place at different sorts of detectors all over the world—and in space—all going after one of the biggest mysteries in science, a theoretical "something" that doesn't fit into current mainstream models of physics, doesn't seem to play into our everyday lives (vs. the Higgs, which is our everyday lives), and, some might argue, doesn't even exist—even though it does make up the vast majority of matter in the universe. But, while still way-cool and far more accurate, the name "dark matter" also might not leap off the page quite like "god particle."

Fortunately, searching for dark matter doesn't require quite the resources as the Large Hadron Collider; if it did, we might not be looking for it at all. Rather than creating and steering high-energy particles into controlled collisions, dark matter researchers just have to detect their prey.

The Cryogenic Dark Matter Search, an experiment located at the bottom of an abandoned mine in Minnesota, only cost about $15 million to build, while the Large Hadron Detector cost somewhere in the neighborhood of $10 billion. The cosmic ray detector aboard the International Space Station, site of some very exciting detection results just a couple of weeks ago, cost around $2 billion, but dark matter is only a part of that project's usefulness.

One good thing about the difference in the two particle hunts is that, with dark matter, there's a lot less waiting around. There are six different direct dark matter detection experiments in the world currently and just as many indirect detection experiments. The nature of dark matter's supposed products—what happens when it has a rare weak interaction with regular matter—lends well to dark matter projects piggybacking on other projects, like the IceCube neutrino detector in Antarctica.


Together, that's a lot of press offices to keep the fires of curiosity stoked. However, those press offices have mostly released a fair amount of negative results so far, deepening the mystery on one hand, but also giving indirect support to alternative theories to dark matter, like MOND, which hypothesizes that Newton's theory of gravity itself is either incorrect or non-universal. (This mostly maligned theory would explain the gravitational effects of dark matter we observe as changes in gravity's effects over distance.)

In terms of results, things started heating up over the past few weeks with, first, those results from the ISS detector and, yesterday, results from CDMS mentioned above. The results from CDMS are actually based on old data compiled from an earlier generation of detection plates designed to register the theoretical effects of a WIMP particle (weakly interacting massive particle, the leading theory for dark matter) colliding with the nucleus of an atom of "normal matter" in conditions kept near absolute zero degrees.


Over 140 days, the team at the Soudan Underground Laboratory registered three events likely to be the result of a WIMP-nucleus collision with a Sigma-3 level of uncertainty. (A "4" counts as evidence, while a "5" a discovery.) So, there remains a slim chance (just a fraction of a percent) that what the researchers at Soudan have observed could have occurred without dark matter.

The energy/mass range of the particle hinted at by these results falls into an interesting slot within dark matter research. With a value somewhere around 8.6 GeV, the CDMS results align well with other results from the CoGeNT dark matter experiment and the Fermi Gamma-Ray Space Telescope. It might be a wash in the end though: that mass also directly contradicts results from the high-profile XENON experiment.


From Fermilab's Symmetry Breaking magazine/blog:

“There’s been an interesting back-and-forth between experiments,” says CDMS Spokesperson Blas Cabrera of Stanford University and SLAC National Accelerator Laboratory. To investigate this hint, “we’ll certainly need more data. If a signal persists, it will need to be replicated by other experiments with different technologies before it is accepted by the community.”

One nice thing about dark matter detectors is that they're relatively easy to upgrade (compared to, say, the LHC). CDMS is in the process of being refitting with heavier germanium detection plates, which are actually what the team there expected to deliver results. The current results actually come from lighter silicon plates that, originally, were only expected to verify results from the germanium plates.

It's possible that we'll see yet another generation of silicon plates later on, only more capable. By then, it's likely the location of CDMS itself will be upgraded to yet another underground ex-mine location, the deeper SNOLAB facility in Sudbury, Ontario. In any case, the dark matter hunt feels a bit more like the cross-country scramble of It's a Mad Mad Mad World than the worldwide manhunts than the worldwide all-in focus of the LHC.

We'll hear plenty more just this year from other experiments and even from CDMS, as yet more data from the silicon plate era is analyzed. As a theoretical physics spectator, you couldn't ask for much more.

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Image: CDMS detector, via Fermilab/Reidar Hahn