Enrico Sacchetti, XENON

The World's Best Dark Matter Detector Just Delivered Its First Results

Finding the Universe's missing 85%.

DAVID NIELD
23 MAY 2017
 

Three years after construction started, the most sensitive dark matter detector ever built has served up its first set of results – and it's working just as its makers intended.

The XENON1T machine, which we first reported on back in 2015, is our best shot yet at finding the notoriously elusive dark matter, which physicists think accounts for more than 85 percent of matter in the entire Universe.

 

While the published results represent only 34 days of data, scientists have confirmed that XENON1T has hit the lowest low-energy background level ever achieved by a dark matter experiment. In other words, if anything can spot the faint ripples of dark matter against the background noise of the Universe, XENON1T can.

It hasn't seen any dark matter yet, but the team behind the project is pleased with the results so far.

"I think the most exciting thing is the fact that the detector works as we expect," one of the researchers, Laura Baudis from the University of Zurich, Switzerland, told Ryan F. Mandelbaum at Gizmodo.

XENON1T is tasked with detecting the interactions between weakly interacting massive particles (WIMPs) – which scientists think could represent dark matter – and regular matter.

The difficulty is in reducing outside interference from natural radioactivity enough to be able to see these very slight jolts if and when they happen.

For that reason, XENON1T is located about 3,600 metres (11,811 feet) below ground, and filled with 2,000 kilograms (4,409 pounds) of liquid xenon – a substance that should spark into life when it gets hit by a WIMP.

 

Particle interactions in liquid xenon cause flashes of light, and scientists are hoping to track those flashes and use other measurements and timings from the chamber to work out whether they could be dark matter.

There's no sign yet from the first month of results, but that's not necessarily a surprise. As Gizmodo explains, it's like putting out a bowl in your back yard and waiting for a meteor to hit it: 34 days with no hit doesn't mean meteors don't exist.

Another study published last year also drew a blank when looking for dark matter, but every extra bit of research is useful. Even if no dark matter is found, it helps narrow down the next search.

The first batch of results have yet to be peer-reviewed, so treat them as preliminary findings for now, but the team is encouraged by what they've seen so far.

"WIMPs did not show up in this first search with XENON1T, but we also did not expect them so soon," says one of the research team, Elena Aprile from Columbia University.

"The best news is that the experiment continues to accumulate excellent data which will allow us to test quite soon the WIMP hypothesis in a region of mass and cross-section with normal atoms as never before."

 

Several other attempts to detect dark matter are underway, whether using different substances to xenon or different equipment altogether, like the Large Hadron Collider (LHC).

We've also seen data hinting at the existence of dark matter collected from the International Space Station (ISS). In fact, it's mostly through observations of deep space that scientists think dark matter exists at all.

The hunt continues, but XENON1T has already proven to be an important new tool for scientists to use in their search.

"A new phase in the race to detect dark matter with ultralow background massive detectors on Earth has just began with XENON1T," says Aprile. "We are proud to be at the forefront of the race with this amazing detector, the first of its kind."

The initial findings have been published on the pre-print website, arXiv.org.

Correction: An earlier version of this story stated that dark matter 'accounts for more than 85 percent of the mass of the entire Universe'. We have now clarified that this percentage applies specifically to matter instead of mass.

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