LUX results: Dark matter hunt nears final phase

By Rebecca Morelle
Science reporter, BBC World Service

image captionThe first results show LUX is the most sensitive detector in the world

Scientists could be nearing the final phase of the search for dark matter: the enigmatic substance thought to make up a quarter of our Universe.

The first results from a particle detector called LUX show it is the most powerful experiment of its kind.

It did not detect any dark matter during its first run, but scientists say it is poised to probe deeper than ever before during its second in 2014.

It has also ruled out earlier hints of dark matter shown by other experiments.

Dr Chamkaur Ghag, a collaborator on the LUX experiment from University College London, said: "If the dark matter is out there and if it interacts the way we think it does we should really start seeing it now."

Not finding any direct evidence for dark matter particles would mean that physicists would have to "go back to the drawing board", he added.

Missing matter

Dark matter is thought to make up 27% of the Universe. But astronomers have only been able to infer its existence through the gravitational effects it has on visible matter in the Universe; no-one has ever directly detected it.

Most scientists believe that it takes the form of particles called Weakly Interacting Massive Particles, or WIMPs, and that millions of these are streaming through us every second without a trace.

There are three main ways that scientists are hunting for this elusive stuff.

One is to look in deep space, using detectors such as the Alpha Magnetic Spectrometer (AMS), to search for the debris that dark matter particles leave behind as they collide with each other.

Another method is with the Large Hadron Collider, where scientists are hoping to create dark matter as they smash particles together.

And the third is using detectors, installed underground, to try and detect the particles as they pass through the Earth. These experiments are set up to spot the very rare occasions when dark matter particles bump into regular matter.

The LUX (Large Underground Xenon) detector is located deep in a mine in the Sanford Underground Research Facility, US. Its first 90-day run took place earlier this year.

"What we've done in these first three months of operation is look at how well the detector is performing, and we're extremely pleased with what we're seeing," said Prof Richard Gaitskell, from Brown University in Rhode Island.

"This first run demonstrates a sensitivity that is better than any previous experiment looking to detect dark matter particles directly."

image captionThe experiment is located deep underground in a gold mine in South Dakota

During this period, the detector did not see any evidence of dark matter.

The team said that this absence also ruled out the possibility that other detectors, such as DAMA in Italy and the CDMS and CoGeNT experiments in the US, had seen glimpses of dark matter.

The researchers said because LUX was so sensitive, if these hints had been correct, then it would have seen dark matter particles too.

However, the scientists are hopeful that in the next 300-day run, which is scheduled to start in early 2014, LUX could be the first experiment to directly detect dark matter.

"This really really should be it. I personally hope we will have detected WIMPs within this decade," said Dr Ghag.

"We should know what a quarter of the Universe really is and LUX should be the one that finds it first."

Commenting on the LUX results, Prof Carlos Frenk from Durham University said: "In the last decade, there have been claims by different groups from around the world who say they have cracked the problem - and some of them have not been received by the community with overwhelming confidence. But in the last few years these claims have really come thick and fast from some very good groups - from some of the best experimentalists in the world.

"What LUX has done is brought some more order into the chaotic world of claims and counter claims. So when they say they rule out previous claims - one has to take them very seriously. But no doubt, more debate will ensue."

He said he was hopeful that LUX might find evidence of dark matter.

"I won't get the champagne out right now. But I will buy some and put it in the fridge."

Prof Anne Green from the University of Nottingham added: "LUX is a big step forward in the field of WIMP direct detection. It's really impressive that they've achieved world leading sensitivity with their first, short data taking run.

"LUX is complementary to indirect detection and collider experiments, such as AMS and the LHC. Because we don't know the exact properties of the WIMP, we don't know which type of experiment will see it first. And to be 100% sure we've detected dark matter we'll want to see consistent signals in different experiments."

If LUX fails to conclusively find dark matter, the team has another chance with an even larger and more sensitive experiment: LUX-ZEPLIN.

But if this cannot see any signs of the elusive particles, then it could be that physicists have got the concept of dark matter wrong and will have to come up with some new theories.

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