This article has been updated to correct a misinterpretation of this null result.
Things can go a bit off-topic at Physics World and recent news about dark matter got us talking about the beauty of the Black Hills of South Dakota. This region of forest and rugged topography is smack dab in the middle of the Great Plains of North America and is most famous for the giant sculpture of four US presidents at Mount Rushmore.
A colleague from Kansas fondly recalled a family holiday in the Black Hills – and as an avid skier, I was pleased to learn that the region is home to the highest ski lift between the Alps and the Rockies.
The Black Hills also have a special place in the hearts of physicists – especially those who are interested in dark matter and neutrinos. The region is home to the Sanford Underground Research Facility, which is located 1300 m below the hills in a former gold mine. It was there that Ray Davis and colleagues first detected neutrinos from the Sun, for which Davis shared the 2002 Nobel Prize for Physics.
Today, the huge facility is home to nearly 30 experiments that benefit from the mine’s low background radiation. One of the biggest experiments is LUX–ZEPLIN, which is searching for dark-matter particles.
Hypothetical substance
Dark matter is a hypothetical substance that is invoked to explain the dynamics of galaxies, the large-scale structure of the cosmos, and more. While dark matter is believed to account for 85% of mass in the universe, physicists have little understanding of what it is – or indeed if it actually exists.
So far, the best that experiments like LUX–ZEPLIN have done is to tell physicists what dark matter isn’t. Now, the latest result from LUX–ZEPLIN places the best-ever limits on the nature of dark-matter particles called WIMPs.
The measurement involved watching several tonnes of liquid xenon for 280 days, looking for flashes of light that would be created when a WIMP collides with a xenon nuclei. However no evidence was seen for collisions with WIMPs heavier than 9 GeV/c2 – which is about 10 times the mass of the proton.
The team says that the result is “nearly five times better” than previous WIMP searches. “These are new world-leading constraints by a sizable margin on dark matter and WIMPs,” explains Chamkaur Ghag, who speaks for the LUX–ZEPLIN team and is based at University College London.
Digging for treasure
“If you think of the search for dark matter like looking for buried treasure, we’ve dug almost five times deeper than anyone else has in the past,” says Scott Kravitz of the University of Texas at Austin who is the deputy physics coordinator for the experiment.
This will not be the last that we hear from LUX–ZEPLIN, which will collect a total of 1000 days of data before it switches off in 2028. And it’s not only dark matter that the experiment is looking for. Because it is in a low background environment, LUX–ZEPLIN is also being used to search for other rare or hypothetical events such as the radioactive decay of xenon, neutrinoless double beta decay and neutrinos from the beta decay of boron nuclei in the Sun.
LUX–ZEPLIN is not the only experiment at Sanford that is looking for neutrinos. The Deep Underground Neutrino Experiment (DUNE) is currently under construction at the lab and is expected to be completed in 2028. DUNE will detect neutrinos in four huge tanks that will each be filled with 17,000 tonnes of liquid argon. Some neutrinos will be beamed from 1300 km away at Fermilab near Chicago and together the facilities will comprise the Long-Baseline Neutrino Facility.
One aim of the facility is to study the flavour oscillation of neutrinos as they travel over long distances. This could help explain why there is much more matter than antimatter in the universe. By detecting neutrinos from exploding stars, DUNE could also shed light on the nuclear processes that occur during supernovae. And, it might even detect the radioactive decay of the proton, a hypothetical process that could point to physics beyond the Standard Model.
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