SYDNEY: There is a 75% chance that scientists have detected dark matter, in the form of weakly interacting massive particles (WIMPs), at an experiment deep underground in Minnesota, USA.

One of the most popular candidates for dark matter - the elusive, invisible substance thought to make up around 85% of the mass of the universe - are weakly interacting massive particles, or WIMPs.

WIMPs are hypothesised dark matter particles that scientists predict could occasionally, although very rarely and weakly, interact with normal matter. When this occurs, the dark matter particles would scatter from an atomic nucleus like billiard balls, leaving behind a small amount of energy detectable under the right conditions.

Detecting WIMP interactions

The Cryogenic Dark Matter Search (CDMS-II) experiment has been searching for WIMPs in detectors located 800 m underground in the Soudan mine in northern Minnesota, USA, since 2003.

In the mine, 30 detectors made of germanium and silicon crystals are cooled to near absolute zero (-273 degrees Celsius) and equipped with special sensors to detect potential WIMP scattering events.

When a particle interacts in the cooled detectors, it releases energy in the form of heat and electric charge. The sensors are then able to detect and record the two signals for later comparison, enabling scientists to tell whether the particle was a WIMP or one of numerous other known background particles, such as solar and cosmic rays.

Two possible detections of dark matter

The results of the latest data set taken from the CDMS-II experiment over 2007 and 2008 and published in the online version of the US journal Science show the detection of two possible WIMP events.

However, in order to claim a discovery there must be less than one chance in a thousand that the detected signal is due to background events. In this case, that would have required the observation of five events.

With just two observed, however, it is estimated that there is a one in four chance that they may have instead resulted from 'background noise'.

Is it just an unlucky fluctuation?

"Either we got an unlucky fluctuation in the number of background events or we were on the edge of a discovery we could not claim," said Jodi Cooley, a particle physicist from Southern Methodist University in Dallas, Texas, who was involved in the experiment.

The researchers conducted the experiment 'blindly' - that is, the data was analysed without looking at the data region that could contain WIMP events.

This is done in order to avoid any unintentional bias that might lead to counting something with the right characteristics as a WIMP, which is in fact the result of background sources.

First blind analysis

Once all of the data selection criteria have been completed, and detailed estimates of background 'leakage' into WIMP signal region are made, the scientists then check to see if there have been any WIMP events. It was at this point that they discovered that two potential dark matter interactions had occurred.

"This is the first time in a blind analysis that we have had events," said Cooley.

The results have also helped to exclude some theories about the way dark matter interacts with normal matter, according to Cooley.

"On the edge of a discovery"

Anne Green, an astroparticle physicist from Britain's University of Nottingham, thinks that while the two events were likely due to background sources, "exciting times are definitely ahead."

"The experiments have got a good chance of detecting WIMPs in the next few years," she said. "However demonstrating that they are definitely due to WIMPs and not backgrounds is going to be a challenge."

The next step in the Cryogenic Search for Dark Matter experiment will be the construction of a much larger WIMP detector called SuperCDMS, which scientists hope to have up and running in the middle of this year.

While the technology of SuperCDMS will rely on the same basic principals, the overall geometry and microscopic design has been changed in order to better distinguish between WIMP and background events.

"Hopefully we are on the edge of a discovery," says Cooley.

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