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A British physicist has led an international team that may have detected the first signal of dark matter, following a single particle interaction recorded deep beneath South Dakota.
Dr Sam Eriksen from the University of Bristol announced the finding today at a scientific conference in Japan, describing it as potentially "the first hint of a dark matter observation".
The LUX-ZEPLIN experiment recorded an interaction that behaves consistently with a WIMP – a weakly interacting massive particle – which represents one of the leading theoretical candidates for dark matter.
However, researchers have emphasised caution. The observation falls short of the statistical threshold required for physicists to declare a confirmed discovery.
The international collaboration involves 250 scientists and engineers from 39 institutions across six countries, including nine British universities.
Dark matter constitutes approximately 85 per cent of all matter throughout the universe, yet scientists have spent nearly a century unable to directly observe it.
The substance neither emits nor reflects light, rendering it completely invisible to telescopes and the human eye. What we can observe – stars, planets and everything visible – comprises merely 15 per cent of universal matter.
Despite its elusive nature, researchers remain confident of dark matter's existence based on observable gravitational effects at galactic scales. Without this gravitational influence, clusters of ordinary matter could not hold together in the cosmos.
The leading hypothesis suggests dark matter consists of particles created during the universe's early formation that persist today.
WIMPs – weakly interacting massive particles – have long represented the foremost candidate, though such particles would interact with ordinary matter extraordinarily rarely.
The experiment operates approximately a mile beneath the surface at South Dakota's Sanford Underground Research Facility, which occupies a former gold mine in the Black Hills region.
Scientists deployed ten tonnes of ultrapure liquid xenon within a large cylindrical detection vessel, managed by the Lawrence Berkeley National Laboratory under the United States Department of Energy.
The detection mechanism searches for rare instances when dark matter might scatter off xenon atoms. Such collisions generate flashes of light, with the characteristics of these luminous signals revealing which type of particle has interacted with the xenon.
Extensive shielding protects the apparatus from interference. A mile of rock overhead blocks cosmic rays from space, whilst a surrounding water tank and outer detector systems guard against background neutrons.
The experiment also employs sophisticated computational methods to distinguish genuine particle interactions from mimics that could produce false signals, enabling researchers to isolate potential dark matter events from ordinary background radiation.
The recorded interaction registered at approximately 248 kiloelectronvolts, significantly exceeding the energy levels scientists typically associate with simple WIMP collisions.
Researchers had initially focused their search on events below 30 kiloelectronvolts, which would indicate straightforward interactions between a WIMP and individual nucleons within the xenon nucleus. The discovery emerged only after the team re-examined their first 220 days of data whilst looking at higher energy ranges.
This elevated energy level suggests a more intricate mechanism at work. Rather than a basic collision, the interaction would have involved coupling between the WIMP and the entire xenon nucleus, according to the analysis.
Such a complex interaction implies the hypothetical particle carries substantial mass – exceeding 200 times that of a proton. The observation also points towards "a specific type of interaction between WIMPs and ordinary matter beyond the simplest model".
The finding achieves a statistical significance of 2.6 sigma, falling considerably short of the 5-sigma threshold that physicists require before declaring a discovery. At the 5-sigma level, the probability of a false signal drops to approximately one in 3.5 million.
Currently, roughly a 0.5 per cent probability exists that known background sources could explain the event. Whilst this might appear reassuringly low, the nature of experimental physics demands exceptional caution.
"Importantly, as it's just a single event, we are not claiming that it is dark matter," Dr Eriksen stated.
Professor Rick Gaitskell from Brown University, the experiment's spokesperson, reinforced this measured approach: "With only one event, we don't want to get ahead of ourselves. We are not claiming to have seen dark matter. But we have seen something interesting that we want to share with the scientific community for their input."
UCLA astrophysicist Alvine Kamaha, a study co-author, added: "We may be looking at something extraordinary, but we have to be exceptionally rigorous before drawing that conclusion.
"The analysis presented covered merely the first 220 days of data collection, representing approximately one-third of the information LUX-ZEPLIN has already gathered. Additional analysis of the remaining data could prove crucial.
Researchers now possess a specific energy range to target in their continued investigation. Multiple dark matter detection facilities operating worldwide maintain their own datasets, which could provide corroborating evidence if similar interactions appear.
Wick Haxton at the University of California, Berkeley, who did not participate in the research, observed: "Before one can declare victory, you need a few more data points, but now they have something to aim for; they know where to look."
Further events detected at comparable energy levels could elevate the statistical significance towards the 5-sigma threshold required for official discovery. Continued data collection will determine whether the signal strengthens or dissipates.
Should the finding prove genuine, it would fundamentally transform physicists' understanding of the universe and could reveal otherwise inaccessible information about cosmic evolution during the early universe.
