The Sciences

Dark Matter Still Hiding

Cosmic VarianceBy Sean CarrollJul 20, 2012 2:07 PM

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After a few provocative hints over the last few years, new results in the search for weakly-interacting dark matter have come up empty. The latest is from XENON100, a liquid-xenon scintillation detector under the mountain in Gran Sasso, Italy. Here are the talk slides by Elena Aprile (pdf) from the Dark Attack conference in Switzerland (via Flip Tanedo). And here's the money plot; dark matter mass is on the horizontal axis, interaction cross section between dark matter and nucleons is on the vertical axis. The colorful bands represent the exclusion limits; anything above that is ruled out.

A couple things to note. The blobs scattered around the plot represent those provocative hints I referred to -- the tentative evidence from previous experiments that they might actually be seeing something. XENON seems inconsistent with all of them. However, you can only make a plot like this under certain theoretical assumptions. Even if those assumptions are quite likely to be true, it's hard to be completely definitive about one experiment ruling out another one, unless they're really using identical techniques (which none of these are). It's possible, although maybe hard to imagine, that some complicated dark-matter physics can make everything consistent. The second point is the dark grey area at the bottom right. That represents a bunch of theoretical predictions in supersymmetric models. As Flip cautions, we don't have a sensible measure on the space of all models, so the blob should be taken as suggestive rather than definitive. But the suggestion is clear: we've ruled out some models, but there are plenty that we haven't yet reached. Progress continues. XENON100 used 150kg of liquid xenon; the plan is to upgrade to one ton. Once that happens, they should be able to improve the limits on the cross section by a factor of 1000, which will swipe into a much larger region of parameter space. We'll see what happens.

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