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Science Sidebar 2: Dark Matter

Another item that didn’t make the paper.

So, dark matter.  It’s stuff.

The comment is prompted by the announcement a little while ago that the US Department of Energy has chosen which three next-generation direct detection experiments it’s going to fund.

So, what is dark matter, and what are these experiments doing, anyway?

This is a doodle I made once of a sparticle, or supersymmetric particle.  Dark matter WIMPs may be sparticles.  (No, it's not an insult, stop waving that spear around.)

This is a doodle I made once of a sparticle, or supersymmetric particle. Dark matter WIMPs may be sparticles. (No, it’s not an insult, stop waving that spear around.)

Cold Dark Matter Search

Dark matter is like the force: it surrounds us and penetrates us, and binds the universe together.  (Thank you, Obi-wan.)

We know dark matter exists because we can see its effects on ordinary matter, which has a convenient tendency to glow in the dark.  We see stars orbiting around their galaxies too fast to be held in place unless there were dark matter we can’t see; we see galaxies orbiting each other, or in clusters, at such a great velocity that they would fly apart… unless there were dark matter.  And on and on.  There’s also evidence for the existence of additional, weakly-interacting matter imprinted on the Cosmic Microwave Background (CMB), the afterglow of the Big Bang.

There’s more evidence than I want to go into right now, but if you’re still skeptical, check out the Bullet Cluster.

The predominant current theory is that dark matter is a weakly interacting massive particle — or WIMP, because physicists can’t resist a good acronym.  Something like a supersized neutrino — bulky instead of super-light, and only interacting very infrequently with normal matter.  We’re inside a galaxy, so there’s conveniently a lot of dark matter around and in us all the time, but it generally doesn’t do much.

This is where CDMS comes in, with SuperCDMS as the successor.  Both versions work with large chunks of germanium or silicon metal, waiting to get lucky and have a dark matter particle slam into one of the nuclei.  Those particular elements are chosen both because they’re a convenient mass (close to the DM particle), and because they have nice properties for detecting collisions.

To avoid confusion with other particles (like neutrons from radioactive decays or cosmic rays or other things like that), the whole experiment has to be way underground.  The same is true for…


Nope, it’s not a band.  And the acronym gets shorted to LZ.  They’re combining two previous experiments into one collaboration.  This one is similar in principle to SuperCDMS, except that it uses liquid Xenon instead of silicon or germanium.  (Cool, right?)

Anyway, the main difference in results is that LZ will be more sensitive to higher mass WIMP-type particles than SuperCDMS is.  But, there is a region of overlap: low-mass LZ may overlap with high-mass SuperCDMS, allowing the two different experiments to cross-check each other.

It’d be cool if dark matter existed there.


This is where stuff gets weird.  Weirder.

This experiment doesn’t try to detect dark matter with atoms.  Instead, it’s using microwaves.  It’s looking for a particle called an axion, which is much less massive, and hypothesized to have some strange properties when it interacts with light.

That particle was postulated to solve a fundamental problem in particle physics related to what’s called “CP violation”, but maybe it can handle the dark matter problem as well.  It’s not as popular as some of the more massive WIMP theories, but it should still be tested.  Since we don’t know what the right answer is yet.

 All The Other Things

Yes, there are lots of other experiments.  I have serious doubts about their claimed detections, though.

Also, it’s quite possible that dark matter is more than one thing.  Hey, maybe it’s two different kinds of WIMP, plus axions and something else we haven’t thought of yet.

And that’s that.  … no, let’s leave dark energy for another day.

  1. michaelbusch
    2014/08/06 at 2:29 pm

    To illustrate how sensitive an experiment has to be to detect dark matter:

    SuperCDMS is the successor to the original Cryogenic Dark Matter Search. The detectors for the original CDMS were buried inside a huge mass of cryogenic refrigerators, shielding, and active cancelation detectors under 800 m of rock in the University of Minnesota’s underground lab at the old Soudan iron mine – all to screen out as much of the background of normal-matter interactions that might be confused with potential WIMP interactions.

    For SuperCDMS, even 800 m of rock is insufficient to appropriately block cosmic rays that might send charged particles into the detector and confuse things. So they’re taking it to SNOLAB, where there is at least 2000 m of rock in all directions around the test chamber.

    Dark matter detection is a serious business.

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