LEAD, South Dakota (AP) — Nearly a mile underground in an abandoned gold mine, one of the most important quests in physics has come up empty-handed in the search for the elusive substance known as dark matter, scientists announced Wednesday.
The most advanced Earth-based search for the mysterious material that has mass but cannot be seen turned up "absolutely no signal" of dark matter, said Richard Gaitskell of Brown University, a scientist working on the Large Underground Xenon experiment. A detector attached to the International Space Station has so far also failed to find any dark matter.
Physicists released their initial findings Wednesday after the experiment's first few months of operation at the Sanford Underground Research Facility, which was built in the former Homestake gold mine in South Dakota's Black Hills.
With 4,580 feet (1,400 meters) of earth helping screen out background radiation, scientists tried to trap dark matter, which they hoped would be revealed in the form of weakly interacting massive particles, nicknamed WIMPS. The search, using the most sensitive equipment in the world, tried looking for the light fingerprint of a WIMP bouncing off an atomic nucleus of xenon cooled to minus 150 degrees Fahrenheit (minus 101 Celsius).
But nothing was found, said co-investigator Daniel McKinsey, a physicist at Yale University. The team plans to keep looking for another year, but McKinsey and Gaitskell were not optimistic about finding dark matter with the current setup. They are already planning to build a more sensitive experiment on the site, using a bigger tank of xenon.
"The short story is that we didn't see dark matter interacting, but we had the most sensitive search for dark matter ever performed in the world," McKinsey said.
The lab, in a bright, clean space at the end of an old mining tunnel filled with pipes and electric cables, is reached by a 10-minute ride in an elevator that once carried miners. Gaitskell and McKinsey said the experiment has far less radiation interference from cosmic rays than any other dark-matter lab.
Essentially, scientists are searching for something they are fairly sure exists and is crucial to the entire universe. But they do not know what it looks like or where to find it. And they are not sure if it's a bunch of light particles that weakly interact or if it is more like a black hole.
"It's ghost-like matter," McKinsey said.
"We are really searching in the dark in a way," said Harvard University physicist Avi Loeb, who is not part of the LUX team. "We have no clue. We don't know what this matter is."
But they keep looking. Gaitskell has been hunting for dark matter for 25 years, originally thinking the effort would take five years. "It's like the pursuit of the Holy Grail, but hopefully this has a different outcome."
Even more so than the recently discovered Higgs Boson, dark matter is central to the universe.
"Dark matter holds every cosmic structure in the universe together," including our own galaxy, said University of Chicago cosmologist Michael Turner, president of the American Physical Society. Turner was not part of the LUX study.
About one-quarter of the universe is comprised of dark matter — five times that of the ordinary matter that makes up everything we see. Dark matter is often defined by what it isn't: something that can be seen and something that is energy.
Researchers are pretty sure dark matter exists, but they are not certain what it is made of or how it interacts with ordinary matter. Dark matter is vital to all the scientific theories explaining how the universe is expanding and how galaxies interact and move.
"We know there's stuff out there that is something else and that makes these searches hugely important because we know we are missing most of the universe," said Neal Weiner, director of the Center for Cosmology and Particle Physics at New York University, who was not part of the search.
Gaitskell and McKinsey said they looked for three "candidate WIMP events" that other teams' experiments hinted at finding. And LUX came up completely empty, indicating that those other experiments must not have found anything.
One of the experimenters, Juan Collar of the University of Chicago, said he wanted to see the details from the LUX results before assessing how it affected his work.
The lack of success could just mean the equipment isn't sensitive enough, so bigger, more sensitive and expensive equipment will be needed, Gaitskell and McKinsey said.
Or it could be, considering the lack of knowledge about what dark matter really is, that "perhaps we're going in the wrong direction," Loeb said.
He said dark matter is most likely a particle and that fits with current theory. But if it is more like a small black hole, physicists will not uncover it in this type of search. Or it may be so small, we cannot find it.
AP Science Writer Seth Borenstein reported from Washington.
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