April 24, 2014 11:59AM
By Seth Borenstein and Chet Brokaw
LEAD, S.D. — 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, or LUX. A detector attached to the International Space Station has so far failed to find any dark matter either.
Physicists released their initial findings Wednesday after the experiment’s first few months at the Sanford Underground Research Facility, which was built in the former Homestake gold mine in South Dakota’s Black Hills.
With more than 4,800 feet 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.
But nothing was found. The team plans to keep looking for another year, but members have doubts 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.
Still, physicists were upbeat, noting that the results eliminated some theoretical candidates for dark matter. And there are many more theoretical models to search for.
“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,” said Daniel McKinsey, a physicist at Yale University.
The LUX experiment was 20 times more sensitive than any previous experiments, they said. The proposed next experiment would be 1,000 times more sensitive still.
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.
Researchers “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.”
Even more so than the recently discovered Higgs boson, dark matter is central to the universe.
About one-quarter of the cosmos 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.
Scientists are pretty sure dark matter exists, but they are not certain what it is made of or how it interacts with ordinary matter.
It is considered vital to all the scientific theories explaining how the universe is expanding and how galaxies move and interact.
“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.
The lack of success could mean the instruments are inadequate, Gaitskell and McKinsey said.
Or, considering the lack of knowledge about what dark matter really is, “perhaps we’re going in the wrong direction,” Loeb said.