Dark matter detector LUX fails to spot particles, still a success | Science| In-depth reporting on science and technology | DW | 31.10.2013
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Dark matter detector LUX fails to spot particles, still a success

The world's most advanced instrument to detect dark matter, a substance which makes up a quarter of our universe, failed to see a single dark particle during a three-month trial. But scientists say it's not a setback.

Buried deep below the ground in an abandoned gold mine in the US state of South Dakota, the Large Underground Xenon (LUX) experiment was tasked with detecting dark matter particles as the elusive substance made its way into the SanfordUnderground Research Facility.

But, over a period of three months, LUX failed to get even a glimpse of these particles. Researchers plan to continue experiments with LUX for about one more year.

"As far as we can tell the dark matter particles are rare enough and they interact so weakly with the detector that we just haven't seen one yet," said Alexander Murphy of the University of Edinburgh, a scientist involved with LUX. Several universities and labs in the USA, the UK and Portugal collaborated on the project.

There have been experiments in the past where scientists thought they might have come across dark matter particles. However, since the LUX detector is highly sensitive and was placed in an extremely quiet environment shielded from radiation, scientists had expected that LUX would have spotted them, too.

'We didn't see much of anything'

"There's only one experiment in the world that actually claims to have signal from dark matter - that's the DAMA experiment in Italy," Daniel McKinsey of Yale University told DW.

Another dark matter experiment carried out in underground labs in Minnesota recently came across three anomalous events as well.

"Now if those three events that they saw were real, then we would have seen something over 1,500 events over that time span in LUX," McKinsey said. "We didn't see much of anything."

LUX physicist Jeremy Mock inspects the LUX detector, which is suspended in a large tank that will soon hold more than 70,000 gallons of ultra pure water (photo: Matt Kapust, Sanford Underground Research Facility)

A tank filled with liquid xenon deep underground is supposed to spot dark matter particles

That makes it quite likely that those sightings in other experiments weren't actual dark matter particles, but rather some radiation interference from cosmic rays.

'Profound impact'

However, even though the experiment didn't detect the desired particles, it still has "profound impact" on the scientific community, according to Greg Landsberg, a scientist at CERN research center.

"It changes our perspective on what dark matter might look like," the scientist who was not affiliated with LUX told DW, adding that the idea dark matter particles are fairly light no longer seems to stand.

"Previously, there have been hints that there may be low-mass WIMP particles [weakly interacting massive particles] and the LUX results pretty conclusively ruled those out. And that's more than just ruling out a few particles," Murphy told DW.

As several groups around the world try to understand what dark matter is, "lots and lots and lots of people have been focusing on these low-mass WIMPs or these tentative detections of low-mass WIMPs," he added. "And it looks like they've all been wasting their time."

How does the universe work?

So far, scientists can only explain about 5 percent of our universe. Of the unknown 95 percent, dark matter makes up a quarter.

"This is one of the major missing pieces in our understanding of the universe," McKinsey said. "It's a huge fraction of the mass of the universe and of our galaxy. If we can detect that it will open the door to understanding new principles about how the universe works."

Yale University physicist Dan McKinsey in the LUX experiment hall (photo: Matt Kapust, Sanford Underground Research Facility)

Physicist McKinsey at the LUX experiment hall some 4,800 feet (1.5 kilometers) below ground

The LUX experiment was composed of a huge tank filled with liquid xenon gas kept at minus 100 degrees Celsius (-148 degrees Fahrenheit) and was equipped with very sensitive light detectors at the top and bottom.

"If these dark matter particles exist, they are the only things that are capable of getting inside and causing little flashes of light," Murphy explained. These flashes of light would indicate dark matter activity.

Over the years, people switched to different technologies, from using silicon detectors and germanium detectors to using normal gas liquids such as liquid xenon, according to Landsberg. "Liquid xenon is now considered to be the most promising new generation of these experiments."

LUX will continue to run for another year trying to spot dark matter. In the meantime, scientists will build a detector they have said will be about 70 times more effective - LUX Zeplin is expected to use a tank filled with seven tones of xenon.

"That would almost be the definitive word on dark matter," Murphy said. "For technical reasons it would be very hard to build another detector after that."

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