WIMPs: A dark matter

By Joseph Milton, FT science intern

The “discovery” of dark matter – the hypothetical, undetectable substance thought to make up around a quarter of the contents of the universe and the majority of the matter in it – has been covered in some newspapers and websites today.

But, while the confirmation of dark matter’s existence would certainly be big news, the media may be over-egging this discovery.

Thought to provide a kind of “gravitational scaffolding” which supports normal matter, dark matter has eluded detection since its existence was first posited by Fritz Zwicky in 1933.

Since 2003 a team of US scientists has been deep underground in the disused Soudan mine, northern Minnesota, working on the Cryogenic Dark Matter Search (CDMS).

Using a bank of 30 detectors made of germanium and silicon, cooled to temperatures very near absolute zero (−273 degrees Celsius), they have been looking for energy signatures indicative of dark matter.

A cryostat (cooled detector chamber) with detector assembly installed at Soudan Mine

A cryostat (cooled detector chamber) with detector assembly installed at Soudan Mine

Physicists think dark matter could be composed of weakly interacting massive particles, or WIMPs for short. In theory, these particles don’t absorb or emit light and have a mass similar to that of an atomic nucleus.

Scientists think WIMPs may occasionally “bounce off” atomic nuclei, leaving behind an energy trace – the signature the scientists have been looking for.

The CDMS team announced yesterday evening that they had detected two events, or energy signatures, consistent with WIMPs “bouncing off” nuclei.

But Pierre Oddone, director of the Department of Energy’s Fermi National Accelerator Lab (Fermilab) near Chicago, stressed that he could not be sure that what they had detected was the signature of dark matter.

“While this result is consistent with dark matter, it is also consistent with backgrounds,” he said, suggesting that both signatures could have been caused by natural radiation, or by cosmic rays from space.

The rationale behind placing the detectors half a mile underground under layers of shielding materials is to suppress such unwanted interference, but these background events are still periodically detected.

During a seminar at Fermilab, Dan Bauer, head of the CDMS, said there was a one in four chance that the detected signatures were background events, and a later statement from the team reiterated that “we can make no claim to have discovered WIMPs.”

What they have discovered is that the frequency of interactions between the elusive WIMPs and atomic nuclei – the “bouncing” events which leave the energy signatures and which the scientists are looking for – may be lower than was previously thought, as theoretically predicted by the mass of WIMPs. To confuse matters further, assumptions about the mass of WIMPs are also theoretical calculations.

These experiments gave the scientists a more accurate estimate of the frequency of such interactions, and the researchers said they might therefore be useful in eliminating a number of other competing theories which attempt to explain dark matter.

They went on to say that they would have to detect at least five events to be sure they were looking at dark matter signatures. To try and achieve this the team will upgrade their detectors next year and continue the search for WIMPs.

It seems the scientists have been considerably more cautious in their statements about the significance of these results than some journalists – quelle surprise.

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Clive Cookson, the FT's science editor, picks out the research that everyone should know about, in fields from astronomy to zoology. He also discusses key policy issues, from R&D funding to science education. He'll cover the weird and wonderful, as well as the serious side of science.