Tag Archives: XENON100

Still no sign of dark matter

The search for dark matter is much more difficult than the search for the Higgs. At least with the Higgs we had an idea where to look: the Higgs is part of the Standard Model of particle physics, after all, and so physicists could guess at least some of its properties. If the Higgs existed, physicists knew they could catch glimpses of it at the LHC.

It’s different with dark matter.

If dark matter particles exist then they clearly and unambiguously relate to physics beyond the Standard Model. This in turn means there are very few clues that can help physicists in the search for dark matter (a search that is incredibly difficult anyway, since dark matter interacts so rarely with “normal” matter). Since physicists by definition don’t know what it is they are looking for when they search for dark matter, that complicates the search enormously: the sort of experiment that can look for axions (one candidate dark matter particle) is very different from the sort of experiment that can look for WIMPs (another candidate).

The currently favoured explanation for dark matter is perhaps the WIMP explanation – that most of the material in the universe consists of Weakly Interacting Massive Particles. Many experiments are currently looking for WIMPs and, as explained in my book New Eyes on the Universe, the results are intriguing. Some experiments seem to have found tentative signs of a WIMP signal; others experiments have found nothing.

Xenon100 photomultipliers

The XENON100 experiment uses arrays of photomultipliers, such as this one, to catch the brief flashes of light that would occur if a WIMP scattered off a xenon nucleus.
(Credit: XENON100 Collaboration)

One of the biggest WIMP dark matter experiments to date as been the XENON100 collaboration, which is based deep underground at the Gran Sasso National Laboratory. The experiment employs 62kg of extremely pure liquid xenon as a WIMP target.The idea is that, once in the proverbial blue moon, a WIMP will score a direct hit on a xenon nucleus and the collision will emit small amounts of light. Sensitive detectors surrounding the liquid can detect light from the collisions.

On 18 July 2012, the collaboration announced the results from an analysis of 13 months of searching. They found no evidence that a WIMP, in all that time, had interacted with a xenon nucleus in their target.

Whatever dark matter is, it’s going to be difficult to find!