Latest search for dark matter draws a blank

While the Large Underground Xenon, or LUX, experiment found nothing, at least we now know a little better what dark matter is not. Cathal O’Connell explains.

Physicists assemble the LUX (Large Underground Xenon) detector deep inside the Homestake mine.
John B. Carnett/Bonnier Corporation via Getty Images

The world’s most sensitive search for dark matter has completed its run, and finished empty-handed.

But, although the results turned out negative, the result is still a landmark in the ongoing saga to find out what the universe is made of.

For starters, it strongly rules out a range of theorised dark matter particles. It also contradicts (and probably overrules) some previous experiments, which had seen some hints of dark matter.

The Large Underground Xenon (LUX) dark matter experiment, a detector set deep down a 1500-metre former gold mine in South Dakota, has been running for three years.

Today, at an international dark matter conference – IDM 2016 in Sheffield, UK – the LUX collaboration announced the result of the past two years of research.

Despite accumulating half a million gigabytes of data, the team have found no hint of dark matter, the mysterious stuff that makes up about 27% of the mass-energy in the universe.

Astronomers have known for decades that the way galaxies spin so fast that they should fling themselves apart does not make sense.

But these strange motions can be explained if there’s a bunch of extra matter in and around the galaxies – “dark matter” that we can’t see – that holds them together.

We know it’s out there. We just don’t know what it is.

But now, thanks to the LUX experiment, we know a little better what dark matter is not.

Most physicists believe that dark matter can be explained by some new kind of particle that doesn’t interact with ordinary matter, and theorists have proposed many possibly candidates.

The modern search for dark matter is a to-and-fro between theorists and experimenters — the theorists who dream up the possible new particles, and the experimenters who search for them.

When a proposed dark matter particle is ruled out by experiment, theorists can bin the idea and go back to the drawing board.

One of the strongest dark matter candidates is the weakly interacting massive particle, or WIMP. These are strange, slow moving particles that would interact with matter only through the weak force (which is responsible for radioactive decay).

'The result is unambiguous data we can be proud of and a timely result in this very competitive field – even if it is not the positive detection we were all hoping for.'

If the WIMP idea is correct, billions of these particles pass through your body every second.

LUX was specifically designed to look for WIMPs, but finished its search without detecting any.

Not that this result kills off the WIMP theory, but it does place the most stringent constraints yet on what dark matter might be. In particular, LUX was most sensitive to WIMPs with a mass in the range of 20 to 100 GeV (for comparison, a proton is about 1 GeV). The dark matter community can now focus their searches outside this range.

The result also firmly rules out some regions where other groups had announced tentative hints.

In 1998 the Italian DAMA collaboration announced the discovery of dark matter. Using 250 kilograms of sodium iodide, their experiment detected an annual variation in the total number of detection events — an increase in summer and a decrease in winter.

The DAMA team claimed this was caused by the Earth moving into a kind of “dark matter wind” once a year in its path around the sun. The CoGeNT detector in Minnesota, running since 2009, has seen a similar variation and so seemed to back up the DAMA result.

Based on these experiments, the LUX experiment was expected to have as many as 1,000 detection events a year. But the first result from the LUX experiment in 2013, based on data from its first 100 days of operation, cast doubt.

At the time, the team announced they had seen “absolutely no events consistent with any kind of dark matter”.

Jim Hanhardt (left) with his son physics graduate student Mark at work helping to convert the Homestake goldmine where he was a miner for 15 years, into the LUX laboratory.

John B. Carnett/Bonnier Corporation via Getty Images

Most physicists aligned with LUX over DAMA and CoGeNT, saying the Italian and Minnesota teams had not done enough to rule out other sources of radiation

Now the LUX team has presented the results from the detector's final 20-month run from October 2014 to May 2016. The new results confirm no detection events for the most sensitive dark matter search in history.

The LUX experiment is comprised of a cylinder filled with 368 kilograms of cooled liquid xenon, surrounded by powerful sensors designed to detect a tiny flash of light if a WIMP collides with a xenon atom within the tank.

Besides being deep underground, the detector is within a 270,000-litre tank of water, also helping shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

One of the legacies of LUX will be a series of pioneering calibration measures which allowed the experiment to reach a sensitivity four times better than the original project goals.

These measures, such as deliberately firing neutrons into the detector to simulate a dark matter particle, and injecting radioactive gases to mimic ambient radioactivity and so account for it, will likely be carried forward to future detectors.

“The result is unambiguous data we can be proud of and a timely result in this very competitive field – even if it is not the positive detection we were all hoping for,” says Simon Fiorucci, a physicist at Lawrence Berkeley National Laboratory and the science coordination manager for LUX.

The team is now focusing on setting up their next generation LUX-ZEPLIN experiment. It will use 30 times more liquid xenon and will be at least 70 times more sensitive. It is due to be operational in 2020.

The question now is whether the particle-smashing experiments at CERN will find dark matter first.

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Cathal O'Connell is a science writer based in Melbourne.
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