LUX-ZEPLIN announces new results in search for dark matter.
27 Aug 2024
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The world's most sensitive WIMP dark matter detector, LUX-ZEPLIN, has announced new results in its search for dark matter. These new results imposes further constraints on WIMP candidates. .

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LZ’s cryostat within the underground surface laboratory at the Sanford Underground Research Facility in South Dakota. RAL in collaboration with LNBL was responsible for the material search and its manufacture. 


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Credit: Matthew Kapust/Sanford Underground Research Facility

​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. New results from the world’s most sensitive dark matter detector, LUX-ZEPLIN (LZ), have narrowed down possibilities for one of the leading dark matter candidates: weakly interacting massive particles, or WIMPs. 

LZ, led by the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), hunts for dark matter from a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota.  STFC’s Particle Physics Department (PPD) was one of the founding members of this collaboration and, along with the STFC Technology Department at Rutherford Appleton Laboratory (RAL), was responsible for delivering the ultra-radio-pure titanium cryostat, calibration system and project management. These contributions have been valuable in helping to build the experiment, producing the world’s best results that explore weaker dark matter interactions than ever been searched before and further limiting WIMP constraints.   

“These are new world-leading constraints by a sizable margin on dark matter and WIMPs,” said Chamkaur Ghag, spokesperson for LZ and a professor at University College London (UCL). He noted that the detector and analysis techniques are performing even better than the collaboration expected. “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.” 

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Members of the LZ team in the LZ water tank after the outer detector installation. 

Credit: Matthew Kapust/Sanford Underground Research Facility​


​​The collaboration 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 experiment's sensitivity to faint interactions helps researchers reject potential WIMP dark matter models that don't fit the data, leaving significantly fewer places for WIMPs to hide. 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 analyze 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.  

“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,” said Scott Kravitz, LZ’s deputy physics coordinator and a professor at the University of Texas at Austin. “That’s something you don’t do with a million shovels – you do it by inventing a new tool.” 


Dr Maurits van der Grinten, a senior researcher in PPD, said:” To ensure these detectors are working at maximum efficiency and capable of detecting rare events, calibration systems are employed extensively between the science runs. RAL was responsible for developing the calibration systems software, hardware, and firmware. These systems lower 3 different sources of known intensities at all angles relative to the photomultipliers, verifying their readings against simulation results. These sources are lowered via fine intricate filaments, with a high pulse laser used to track their depth. Many of these specialised components were designed and manufactured at RAL, including the lowering filaments, surrounding materials, and FPGA boards.  The dark matter team at RAL can change the source positioning from across the Atlantic Ocean to a millimetre precision. This feature was not present in the original design but has been added recently. It is useful in providing round-the-clock data collection and calibration, which is instrumental in the detector’s seamless operation.” 

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Photograph of the LZ calibration system. (STFC). 

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 everyday objects, the thousands of components that make up LZ must be made exclusively of ultra-radiopure material. To ensure no interference, each component, down to each washer and screw, had to undergo a thorough selection and precision cleaning. In collaboration with LBNL, RAL led the manufacture and material searches for the cryostat, a colossal double-walled vessel containing 10 Tonnes of liquid xenon at –100°C. Having worked with liquid xenon detectors for over 20 years, Prof. Pawel Majewski, the group leader of dark matter and rare event searches for RAL, said, ‘The reason we were able to get this yet again world’s best result is mainly because of our fantastic data analysis team, the design of the experiment and a rigorous ultra-radiopure material selection program, including commercially ultra radiopure titanium selected after a 2-year screening campaign. The titanium cryostat was one of the main UK deliverables to the LZ project.


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 Photograph of the LZ crysotat. Image credit: Matt Kapust Sanford Underground research facility

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. ​“We’re pushing the boundary into a regime where people have not looked for dark matter before,” said Scott Haselschwardt, the LZ physics coordinator and a recent Chamberlain Fellow at Berkeley Lab who is now an assistant professor at the University of Michigan. “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, a dense transparent medium 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.  

“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,” said Amy Cottle, lead on the WIMP search effort and an assistant professor at UCL. “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.” 

 

UK researchers from PPD, the universities of Bristol, Edinburgh, Liverpool, Oxford, Sheffield, Imperial College and University College London have made major contributions to LZ, along with 37 other institutions from the United States, United Kingdom, Portugal, Switzerland, South Korea, and Australia. Early career researchers played a key role in this outstanding experiment result, the world leading sensitivities would not have been attained without their contributions in construction, operation and analysis. The collaboration is already looking forward to analyzing 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. 

"Our ability to search for dark matter is improving at a rate faster than Moore’s Law,” Kravitz said. “If you look at an exponential curve, everything before now is nothing. Just wait until you see what comes next.” 


​This is an adapted press release specific to the particle physics department of STFC, the original version can be found here. ​​​​​


Contact: Gregory, Kai (STFC,RAL,PPD)