Researchers led by the University of Bristol have identified a mysterious particle interaction deep beneath the United States, and while they are emphatically not claiming the discovery of dark matter, it is the most compelling signal produced by the LUX-ZEPLIN experiment so far.
British scientists are at the forefront of one of the most tantalising searches in modern physics after a UK-led team identified an unexplained particle interaction that could, just possibly, offer a glimpse of the mysterious dark matter believed to permeate our Universe.
The intriguing result comes from the LUX-ZEPLIN, or LZ, experiment, one of the world’s most sensitive attempts to directly detect dark matter.
The analysis was led by scientists at the University of Bristol, with researchers from institutions across Britain playing important roles in the wider international project.
Scientists are rightly being cautious. One unexplained event is nowhere near enough to announce that humanity has finally detected dark matter.
But it is enough to make physicists sit up and take notice.
Dark matter is one of the greatest mysteries in modern science. It is thought to account for around 85 per cent of all matter in the Universe, yet nobody has ever directly detected it.
Scientists infer its existence from the gravitational effects it appears to exert on galaxies and other structures, but precisely what it is made from remains unknown.
One leading possibility is a class of hypothetical particles known as weakly interacting massive particles, or WIMPs.
Finding one would represent an extraordinary scientific breakthrough.
That is exactly what LZ has been built to look for.
The experiment is located nearly a mile underground at the Sanford Underground Research Facility in South Dakota, where the enormous amount of rock above helps shield the detector from unwanted radiation and other interference.
At its heart are 10 tonnes of ultrapure liquid xenon, designed to detect incredibly rare interactions that could reveal the presence of particles currently invisible to science.
And now researchers have found something they cannot readily explain.
A detailed analysis led by the University of Bristol identified a single particle interaction in a region of the data where scientists had not previously searched in this way.
The team then spent months investigating whether a known background process could account for it.
So far, none has provided a satisfactory explanation.
Sam Eriksen, senior research associate at the University of Bristol and lead author of the study, said the scientists’ extraordinarily detailed understanding of the detector meant even one unusual event deserved attention.
“We understand our detector and the backgrounds so well that even a single outstanding event, like the one we found, is important,” he said.
“We expect dark matter events to be extremely rare, so only a handful could mark the first detection of WIMP dark matter.”
The significance of the result currently stands at 2.6 sigma. Researchers estimate there is roughly a 0.5 per cent probability that known background processes could account for the observation.
That might sound compelling, but particle physicists set an exceptionally high bar before declaring a discovery.
The accepted standard is 5 sigma, meaning this result falls well short of the level required to say dark matter has been detected.
If the mysterious event ultimately does prove to have been caused by dark matter, however, the implications could be profound.
Researchers say it would point towards a WIMP with a mass of at least 200 GeV/c², more than 200 times the mass of a proton, interacting with ordinary matter in a way that goes beyond the simplest theoretical models.
For now, the scientists are making no such claim.
Professor Rick Gaitskell, of Brown University, spokesperson for the LZ experiment, said researchers were “very intrigued” because the event had appeared in a region where dark matter might be expected and competing background events are extremely low.
But he stressed, “We are not claiming to have seen dark matter.”
Instead, the international team is publishing the finding so other scientists can examine it while LZ continues gathering evidence.
That caution is science working exactly as it should.
And behind this fascinating result lies an impressive British contribution.
Around 250 scientists and engineers from 39 institutions around the world participate in LZ, with approximately 50 UK researchers contributing to its operation, scientific analysis and leadership.
Ten British institutions are centrally involved, the University of Bristol, the University of Edinburgh, Imperial College London, King’s College London, the University of Liverpool, the University of Oxford, Royal Holloway, University of London, the University of Sheffield, University College London and the Science and Technology Facilities Council’s Rutherford Appleton Laboratory.
Britain’s involvement stretches far beyond analysing the latest result.
UK researchers and engineers helped design, build and test crucial parts of the experiment, while STFC national laboratories and the Boulby Underground Laboratory have contributed expertise and technology to the project.
The UK’s participation in the construction and operation of LZ has been funded by the Science and Technology Facilities Council.
Professor Henrique Araújo, from Imperial College London and STFC’s Particle Physics Department, who leads the LZ project in the UK, paid tribute to the Bristol researchers after years of painstaking work.
“Many in the UK team contributed to this analysis, but clearly the Bristol group had to endure the level of scrutiny one reserves to such a big result,” he said.
He added that Britain’s position at the forefront of the research was possible because UK scientists and engineers had helped “design and build a fantastic instrument”.
And this could be only the beginning.
LZ will continue operating beneath South Dakota, collecting more data that should eventually help researchers establish whether this tantalising signal becomes stronger, fades into statistical insignificance, or is ultimately explained by another phenomenon.
The UK is also looking towards the next generation.
British researchers are already collaborating internationally on XENON-LUX-ZEPLIN-DARWIN, a proposed successor observatory intended to push the hunt for dark matter, and the study of neutrinos, to even greater levels of sensitivity.
Intriguingly, Britain could ultimately become its home.
The project has received preliminary support through the UKRI Infrastructure Fund, while the possibility of locating the experiment at Britain’s own Boulby Underground Laboratory is being explored.
Professor Pawel Majewski, Dark Matter Group Leader at STFC’s Particle Physics Department and an LZ co-investigator, said the progress made with liquid xenon technology demonstrated the case for an even larger experiment.
Future discoveries, he suggested, could be “literally around the corner”.
Nobody should reach for the champagne just yet.
The mystery of dark matter has defeated some of the finest minds in physics for decades, and one unusual particle interaction does not solve it.
But somewhere almost a mile beneath the hills of South Dakota, one event has appeared that scientists cannot yet satisfactorily explain.
And it is a team led from Bristol, supported by an extraordinary network of British scientific talent, that has carried out the painstaking analysis bringing it to the world’s attention.
Britain has a proud history of asking some of science’s biggest questions, and providing some of its greatest answers.
Whether this tiny, mysterious signal eventually turns out to be dark matter remains to be seen.
But once again, British scientists are helping to lead the search.
Image: For illustration purposes only.
