By Will Dunham
Sept 1 (Reuters) – Scientists have made a possible breakthrough in the hunt for dark matter — observing hints of it during an experiment about a mile (1.6 km) underground in South Dakota — though they stopped short of saying they have finally detected this elusive cosmic component.
The researchers described a particle interaction documented at the Sanford Underground Research Facility — built in a former gold mine in the state’s Black Hills region — that may have involved a hypothesized particle called a WIMP, short for “weakly interacting massive particle.” This is one of the proposed candidates for being a dark matter particle.
Ordinary matter makes up stars, planets, people and everything else we can see. But it represents only about 15% of all the matter in the universe. The rest is thought to be dark matter, which does not emit or reflect light, making it invisible to the human eye and to telescopes. But scientists are confident dark matter exists because of its gravitational effects at galactic scales.
Particle physicist Sam Eriksen of the University of Bristol in England, lead author of a study describing the work, said it “could be the first hint of a dark matter observation.”
Dark matter is thought to interact only extremely rarely with ordinary matter. The ongoing LUX-ZEPLIN, or LZ, experiment in which the observation occurred is designed to catch such interactions and is optimized to look for WIMPs.
It employs 10 tons of the chemical element xenon in liquid form inside a detector device — a large cylindrical vessel — that is managed by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory.
The researchers are searching for dark matter scattering off xenon atoms. When a particle scatters, they would observe flashes of light, with the properties of these flashes revealing what type of particle interacted with the xenon.
In an announcement made on Tuesday at a scientific conference in Japan and described in the study submitted to the journal Physical Review Letters, they described the detection of an interaction between a xenon atom and another particle that appeared to unfold in a way hypothesized for a WIMP.
They said what they may have observed was a WIMP colliding with a xenon atom’s nucleus, transferring a small amount of energy that produced a faint flash of UV light that was detected in the experiment. And the collision caused the xenon atom’s nucleus to jolt forward in what is called a nuclear recoil.
‘JUST A SINGLE EVENT’
While the researchers said the interaction acted as they would expect a WIMP to act, they cautioned that the observation does not yet meet the statistical threshold needed to claim a discovery of dark matter.
“Importantly, as it’s just a single event, we are not claiming that it is dark matter,” Eriksen said.
The researchers are working to rule out other explanations.
“We may be looking at something extraordinary, but we have to be exceptionally rigorous before drawing that conclusion,” UCLA astrophysicist and study co-author Alvine Kamaha said.
Scientists are unsure of the nature of dark matter, just as they are uncertain about the nature of the mysterious cosmic force called dark energy.
“There could be millions of dark matter particles passing through our bodies every second, and yet almost none of them will ever interact with an atom in our bodies,” Kamaha said.
A leading hypothesis is that dark matter is made of a type of particle produced in the early universe and still present today.
“When we observe galaxies and clusters of galaxies, we see that they behave as though they contain much more mass than the matter we can see. So although we cannot see dark matter itself, we can observe what its gravity does,” Kamaha said.
“You can think of dark matter as the cosmic glue that helped enable the formation of galaxies like our Milky Way. Without dark matter, the universe would have evolved very differently, and the structures that eventually led to our solar system may not have formed in the same way,” Kamaha said.
Scientists are searching for dark matter in several ways.
“We study its gravitational effects on galaxies and the universe. We try to produce dark matter particles in particle accelerators. And we perform direct detection using experiments buried deep underground, like LZ, to shield them from cosmic rays and other sources of background while we look for the extremely rare occasion when a dark matter particle interacts with ordinary matter,” Kamaha said.
(Reporting by Will Dunham in Washington; Editing by Daniel Wallis)
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