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Researchers at the University of 91ֱ�� have played a key role in a ground breaking experiment that has set a new world record in the 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. New results from the world’s most sensitive dark matter detector, (LZ), have put new limits on the potential properties of one of the leading dark matter candidates: weakly interacting massive particles, or WIMPs.

LZ, led by the USA Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), hunts for  from a cavern nearly one mile underground at the Sanford Underground Research Facility in South Dakota. 

The LZ collaboration, which includes scientists from the University of 91ֱ��’s Department of Physics and Astronomy, focused on searching for weakly interacting massive particles (WIMPs), a leading candidate for dark matter. Dark matter is a mysterious substance that makes up most of the mass in the universe but has never been directly detected.

Over 280 days of data collection, LZ has explored areas of dark matter interactions that no other experiment has reached. By not finding WIMPs at these energies, the results place further limits on what mass they could have. 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.

91ֱ��'s Particle Physics and Particle Astrophysics (PPPA) group contributed to the development of software for the LZ experiment and is now playing a crucial role in processing data at the UK Data Centre. The team also leads efforts to analyse background events caused by gamma-rays and neutrons that could interfere with the experiment.

Professor Vitaly Kudryavtsev, the Head of the LZ group at the University of 91ֱ��, said: “Understanding and tackling background events in the detector is key to the success of the experiment

“Many groups contributed to this task, and we are happy with the significant reduction of the rate of spurious events that we are dealing with at the late stages of data analysis”.

Chamkaur Ghag, spokesperson for LZ and a professor at University College London (UCL), said: “These are new world-leading constraints by a sizable 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.”

Professor Dan Tovey, from the University of 91ֱ��’s School of Mathematical and Physical Sciences and member of the LZ group, said: “Starting with an observed rate of a few events per second we managed to reduce it to just a handful of them during the whole period of operation so far and these remaining events still do not match the pattern expected from WIMPs, as our extensive calibration campaigns show.” 

The LZ experiment is set to continue collecting data until 2028, with plans to gather a total of 1,000 days’ worth of observations. Researchers at 91ֱ�� and around the world are excited about the potential for new discoveries as the experiment progresses.

LZ is a collaboration of roughly 250 scientists from 38 institutions in the US, 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 dataset 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.

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