LZ experiment sets new record in search for dark matter

Figuring out the nature of dark matter, the invisible substance that makes up most of the mass in our universe, is one of the greatest puzzles in physics.

LZ, the world’s most sensitive dark matter detector, found no evidence of WIMPs above a mass of 9 gigaelectronvolts/c² (GeV/c²). (For comparison, the mass of a proton is slightly less than 1 GeV/c².)  The new result, based on 280 days of data, is nearly five times better than the world’s previous best published result.

LZ hunts for dark matter from a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota.

Researchers at UCL play key roles in the collaboration, leading the experiment and primary physics with both the international spokesperson and the WIMP search analysis lead. The experiment’s new results explore areas never searched before and further limits what WIMPs could be.

Professor Chamkaur Ghag, the spokesperson for LZ based at the UCL Department of Physics & Astronomy, said: “These are new world-leading constraints by a sizeable margin on dark matter and WIMPs.

“If WIMPs had been within the region we searched, we’d have been able to robustly say something about them. We know we have the sensitivity and tools to see whether they’re there as we search lower energies and accrue the bulk of this experiment’s lifetime.”

The new results were presented at two physics conferences on August 26: TeV Particle Astrophysics 2024 in Chicago, Illinois, and LIDINE 2024 in São Paulo, Brazil. A science paper will be published in the coming weeks.

The results analyse 280 days’ worth of data: a new set of 220 days (collected between March 2023 and April 2024) combined with 60 earlier days from LZ’s first run. The experiment plans to collect 1,000 days’ worth of data before it ends in 2028. 

Professor Scott Kravitz, LZ’s deputy physics coordinator based at the University of Texas at Austin, said: “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.

“That’s something you don’t do with a million shovels – you do it by inventing a new tool.”

LZ’s sensitivity comes from the myriad ways the detector can reduce backgrounds, the false signals that can impersonate or hide a dark matter interaction. Deep underground, the detector is shielded from cosmic rays coming from space.

To reduce natural radiation from LZ’s thousands of detector parts the UCL group co-led the meticulous and near decade-long campaign to select materials from which to construct LZ, deploying cutting-edge radio-assay facilities at UCL and the UK’s Boulby Underground Laboratory to achieve unprecedented low-radioactivity.

The LZ detector is built like an onion, with each layer either blocking outside radiation or tracking particle interactions to rule out dark matter mimics.

The WIMP search, led by Dr Amy Cottle (UCL Physics & Astronomy), deploys sophisticated analysis techniques to identify potential dark matter candidate events amidst any residual backgrounds, including novel innovation to identify the most common culprit: radon.

Dr Cottle said: “This is a complex analysis, engaging hundreds of scientists over several years to complete thousands of detailed tasks.

“UCL effort has been integral to this work, leading key areas of the analysis and driving innovation to mitigate backgrounds and develop new and powerful statistical inference tools.”

This result is also the first time that LZ has applied “salting” – a technique that adds fake WIMP signals during data collection. By camouflaging the real data until “unsalting” at the very end, researchers can avoid unconscious bias and keep from overly interpreting or changing their analysis.

Dr Scott Haselschwardt, LZ physics coordinator based at the University of Michigan, said: “We’re pushing the boundary into a regime where people have not looked for dark matter before. 

“There’s a human tendency to want to see patterns in data, so it’s really important when you enter this new regime that no bias wanders in. If you make a discovery, you want to get it right.” 

Dark matter, so named because it does not emit, reflect, or absorb light, is estimated to make up 85% of the mass in the universe but has never been directly detected, though it has left its fingerprints on multiple astronomical observations. We wouldn’t exist without this mysterious yet fundamental piece of the universe; dark matter’s mass contributes to the gravitational attraction that helps galaxies form and stay together. 

LZ uses 10 tonnes of liquid xenon to provide a dense, transparent material for dark matter particles to potentially bump into. The hope is for a WIMP to knock into a xenon nucleus, causing it to move, much like a hit from a cue ball in a game of pool. By collecting the light and electrons emitted during interactions, LZ captures potential WIMP signals alongside other data. 

UCL PhD students Emily Perry (now a Chamberlain Fellow at the Lawrence Berkeley National Laboratory), Isabelle Darlington, Simran Dave, and postdoctoral researcher Dr Aiham Al Musalhi were core to the data analysis for the new results - developing event selection routines, identifying and modelling backgrounds across the detector with Monte Carlo simulations, and, with UK colleagues, implementing the radon identification for this search. 

UCL researchers Dr Robert James (now at the University of Melbourne) and Dr Joe McLaughlin led the statistical inference of the data for this analysis, deploying entirely new and revolutionary tools which Dr James developed at UCL with UK colleagues.

UCL PhD student Jacopo Siniscalco was a core developer of the statistical framework, whilst also aiding development of the key data pipeline for the experiment, from the detector underground to computationally intensive data processing and staging for analysts at LZ’s two data centres, one in the US and one in the UK. 

Professor David Waters, Head of High Energy Physics at UCL, said: “Understanding the puzzle of dark matter is one of the most pressing challenges in particle physics today. In addition to the pioneering work of Cham and Amy, we have teams of people at UCL searching for the direct production of dark matter particles at CERN's Large Hadron Collider, mapping dark matter from cosmological surveys, and exploring the theoretical underpinnings of the gravitational evidence for dark matter.

“It’s an incredibly exciting period in this quest, with UCL playing a prominent role.”

Dr Cottle said: “We’ve demonstrated how strong we are as a WIMP search machine, and we’re going to keep running and getting even better – but there’s lots of other things we can do with this detector.

“The next stage is using these data to look at other interesting and rare physics processes, like rare decays of xenon atoms, neutrinoless double beta decay, boron-8 neutrinos from the sun, and other beyond-the-standard-model physics. And this is in addition to probing some of the most interesting and previously inaccessible dark matter models from the last 20 years.”

LZ is a collaboration of roughly 250 scientists from 38 institutions, led by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory, in the United States, UK, Portugal, Switzerland, South Korea, and Australia; much of the work building, operating, and analysing the record-setting experiment is done by early career researchers. The collaboration is already looking forward to analysing the next data set and using new analysis tricks to look for even lower-mass dark matter. Scientists are also thinking through potential upgrades to further improve LZ, and planning for a next-generation dark matter detector called XLZD.

LZ is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics and the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. LZ is also supported by the UK’s Science & Technology Facilities Council; the Portuguese Foundation for Science and Technology; the Swiss National Science Foundation, and the Institute for Basic Science, Korea. Over 38 institutions of higher education and advanced research provided support to LZ.

Links

• LZ dark matter experiment
• Professor Chamkaur Ghag’s academic profile
• Dr Amy Cottle’s academic profile
• UCL Physics & Astronomy
• UCL Mathematical & Physical Sciences

Image

• LZ’s central detector, the time projection chamber, in a surface lab clean room before delivery underground. Credit: Matthew Kapust/Sanford Underground Research Facility

Media contact

Mark Greaves

m.greaves [at] ucl.ac.uk

+44 (0)20 3108 9485