World's most sensitive detector fails to find dark matter

The world's most sensitive dark matter detector has failed to yield any trace of the elusive substance thought to account for more than four-fifths of the mass of the universe, even after 20 months of operation.

The Large Underground Xenon (LUX) dark matter experiment, which operates beneath a mile of rock at the Sanford Underground Research Facility in the Black Hills of South Dakota, has completed its silent search for the missing matter of the universe.

LUX's sensitivity far exceeded the goals for the project, scientists said, but yielded no trace of a dark matter particle.

Its extreme sensitivity makes the team confident that if dark matter particles had interacted with the LUX's xenon target, the detector would almost certainly have seen it.

That enables scientists to confidently eliminate many potential models for dark matter particles, offering critical guidance for the next generation of dark matter experiments.

"LUX has delivered the world's best search sensitivity since its first run in 2013," said Rick Gaitskell, professor at Brown University in the US.

"With this final result from the 2014 to 2016 search, the scientists of the LUX Collaboration have pushed the sensitivity of the instrument to a final performance level that is four times better than the original project goals," Gaitskell said.

"It would have been marvellous if the improved sensitivity had also delivered a clear dark matter signal," he said.

Dark matter is thought to account for more than four-fifths of the mass in the universe.

Scientists are confident of its existence because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe, but experiments have yet to make direct contact with a dark matter particle.

The LUX experiment was designed to look for weakly interacting massive particles (WIMPs), the leading theoretical candidate for a dark matter particle.

If the WIMP idea is correct, billions of these particles pass through your hand every second, and also through the Earth and everything on it.

However, because WIMPs interact so weakly with ordinary matter, this ghostly traverse goes entirely unnoticed.

The LUX detector consists of a third-of-a-tonne of cooled liquid xenon surrounded by powerful sensors designed to detect the tiny flash of light and electrical charge emitted if a WIMP collides with a xenon atom within the tank.

The detector's location at Sanford Lab beneath a mile of rock, and inside a 72,000-gallon, high-purity water tank, helps shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

The 20-month run of LUX represents one of the largest exposures ever collected by a dark matter experiment, the researchers said.