Built on Facts

WIMPy Physics.

Some of you may have heard in the news recently about a possible detection of the particles that may make up dark matter:

Detectors in the mine, part of the Cryogenic Dark Matter Search experiment, were tripped recently by what might be weakly interacting massive particles, or WIMPs. WIMPs are among the most popular candidates for dark matter, the invisible material that scientists think makes up more than 80 percent of the mass in the universe. Recently detectors in the mine recorded two hits with “characteristics consistent with those expected from WIMPs,” according to a statement posted on the Cryogenic Dark Matter Search Web site. There is a one-in-four chance, however, that the particles detected are not dark matter but ordinary subatomic particles such as neutrons, the team cautions.

Long story short, thanks to careful observations of the motions of stars within galaxies, we know that there’s a lot more mass distributed within and around galaxies than can be accounted for by the stars themselves. After decades of work, most of the obvious possibilities have been eliminated. The main suspect left standing is very weakly interacting particles. Because their interactions are so weak, they’re very difficult to detect. Numerous experiments are in progress searching for these particles, but because the universe is also filled with lots of particles that do strongly interact it’s difficult to tease the tiny signal from the occasional WIMP from the background noise. We don’t know much about WIMPs though since we’ve never seen them, and the possible properties they might have could easily render them invisible to these less-sensitive detectors.

But this current iteration of experimentation has detected two possible candidates for WIMP interaction events. The expected background count (ie, false positives) is about 0.6. So it’s a close-run thing, and both events could well be illusory.

The research team’s statistical methods are very solid. They deliberately set up their calibrations and detection criteria before the data collection period in order to have a “double blind” test – making sure to avoid the possibility of biasing the calibration criteria to include specific events. Though it hasn’t been publicized as much, there are in fact four events that might plausibly be WIMP detection, where two of the events fall just outside the rejection criteria. Including them would mean 4 events of an expected ~1.5, though this is not scientifically acceptable since selection bias in the criteria is no longer definitely eliminated. (Widening the criteria farther doesn’t buy any more events until the criteria are really implausibly wide.) Still, it’s an interesting sign.

So on the Mythbuster scale of Confirmed/Plausible/Busted, I’d rate this detection event Plausible. The team conducting the research stresses that this is only a hopeful first step and not anything close to conclusive. Further data runs, including new data runs with better equipment in even better shielded locations, will help move the WIMP detection into the Confirmed or Busted category.

It would really nice to be able to bag a WIMP. The dark matter mystery has been an open question for a long time, and finally putting it to bed will be a tremendous victory.

UPDATE: Ethan of Starts with a Bang (a very excellent physics blog) has a scathing and well-argued contrary take that you should read. He thinks 2 events out of 0.6 is a load of bull. And he’s got a point. But while a bracing dunk in cold water is always good for science, in this case I think the CDMS data is a little more suggestive than he gives it credit. “Plausible” in my Mythbuster ranking means just what is does on the show – just maybe, given further testing. In either case both of us are in complete agreement with the CDMS experimenters: “We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs.”


  1. #1 Ethan Siegel
    December 18, 2009


    You can look at the statistical significance of this.

    In this sort of field (which fall under “experimental particle physics”), you need a three-sigma result to be considered at all legitimate, a four-sigma result to get people to take you seriously, and a five-sigma result for most people to start to accept it.

    This result is not even a two-sigma result in terms of statistical significance. That is why I think it’s a load of sensationalized nothing. One or both of them may be dark matter, but remember what this “one-in-four” thing means. It doesn’t mean that there’s a 1-in-4 chance that this is background. It means that if you took this much data for this long, there’s a 1-in-4 chance that you’d see two background events.

    But as far as new physics goes, I’m an admitted conservative.

  2. #2 dWj
    December 18, 2009

    What would be the nature of these WIMPs, if that’s what they were? Do we have clear measures of mass, spin, decay products, etc?

  3. #3 Blind Squirrel
    December 20, 2009

    The Tower Sudan mine is a state park in northern Minnesota and the location of the U of M’s neutrino telescope. You can tour it! They lower you down 1/2 mile in the original mine elevator! Well worth the trip and definitely not for the claustrophobic. There is usually a free day in the spring.


  4. #4 CCPhysicist
    December 20, 2009

    Let’s wait and see what they get when they double the run time. Two more and they might have something interesting.

    I agree that it would be nice if it was real, but it would also have been nice if that clear signal seen by Blas Cabrera a few decades ago had been a magnetic monopole. And maybe it was, but it has never been seen since so the monopole remains unconfirmed.

  5. #5 Bob Sykes
    December 24, 2009

    “thanks to careful observations of the motions of stars within galaxies, we know that there’s a lot more mass distributed within and around galaxies than can be accounted for by the stars themselves”

    Of course, this statement is true only if Einstein’s General Theory of Relativity is true. Prof. John Moffat and others beg to differ.

    Both Dark Matter and Dark Energy have somewhat of the character of epicycles. That is, they are needed to save the appearances. The fact that quantum mechanics and general relativity are incompatible with each (and we’re no closer than 1920 to solving this problem), means that at least one and most probably both are wrong. It is entirely possible that a correct theory would dispense with Dark Matter and Dark Energy, which would be relegaed to the phlogiston junk pile.

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