The world’s most sensitive dark matter detector is up and running, ready to tackle one of the most perplexing mysteries of the universe. With a sensitivity at least 50 times greater than its predecessors, the LUX-ZEPLIN (LZ) experiment is lurking quietly a mile underground, waiting for signs of these hypothetical particles.
For the better part of a century, scientists have realized that our observations of the universe don’t match what the Standard Model predicts. There’s mounting evidence for a huge amount of invisible matter that influences the stuff we can see through gravity. But frustratingly, this so-called “dark matter” continues to elude direct detection.
And it’s not from a lack of trying. Over the decades many experiments have been searching for signals in various ways, without a peep. Null results aren’t a total washout though – each one helps rule out candidate particles with certain masses or other properties, narrowing the hunting ground for the next generation of dark matter detectors.
And now the latest generation is firing up. The LUX-ZEPLIN experiment is, as the name suggests, the successor to two previous experiments, LUX and ZEPLIN, but this one is at least 50 times more sensitive to possible dark matter signals than its forebears.
LZ is hunting for a specific type of dark matter candidate called weakly interacting massive particles (WIMPs), which are hypothesized to have been created in the early universe and would still be hanging around today. If they are, they would interact with regular matter through gravity and the weak nuclear force, producing the astronomical anomalies associated with dark matter.
As they drift around the cosmos, these WIMPs mostly ignore normal matter, passing right through entire planets and even us. But occasionally, one should bump into the nucleus of an atom, producing a signal that’s detectable with the right equipment.
And LZ is the right equipment. The experiment involves a huge volume of atoms for WIMPs to crash into, surrounded by detectors to watch for any such events. The target is a tank of ultra-pure liquid xenon, and if disturbed by a wandering WIMP it gives off a flash of light and knocks an electron loose, both of which can be picked up by sensors in the tank. The facility is built 1.5 km (0.9 miles) underground and encased in a big tank of water, to shield against other particles like neutrons that produce false positives.
This is the same basic setup that LUX and ZEPLIN both used, but LZ features a few new tricks. For one, the tank itself is much bigger – where the original LUX used 370 kg (816 lb) of xenon for active detection, LZ packs 7 tonnes (7.7 tons), greatly increasing its sensitivity.
To help separate events produced by boring old particles from potentially groundbreaking WIMP detections, the outer water tank also contains a new array of sensors that can detect when known particles are zipping through. That way, any events in the xenon that are accompanied by signals in the water can be ruled out as dark matter detections.
All up, these advances make LZ over 50 times more sensitive than its predecessor experiments, and let it claim the current title of the world’s most sensitive dark matter detector.
LZ operated for a test run of 60 days starting in December 2021, and in that time it didn’t detect any excess of events above the expected background noise. But that’s just the beginning, with the experiment expected to collect data for 1,000 days over its lifetime.
Perhaps by the time it’s done, we may finally have an answer to this cosmic mystery.
The data from the test run was published in a preprint article on ArXiv.
Sources: Sanford Underground Research Facility, LUX-ZEPLIN, Imperial College London
