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Dark matter test detected nothing, researchers say

NEW YORK — The former Homestake Gold Mine in Lead, S.D., has a hallowed place in the history of physics as a spot where nothing happens.

It was there, in the 1970s, that Raymond Davis Jr. attempted to catch neutrinos, spooky subatomic particles emitted by the sun, in a vat of cleaning fluid a mile underground and for a long time came up empty. For revolutionizing the study of those particles, he shared the Nobel Prize in Physics in 2002.

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On Wednesday, an international team of physicists based in the same cavern of the former mine announced a new milestone of frustration, but also hope — this time in the search for dark matter, the mysterious, invisible ingredient that astronomers say makes up a quarter of the cosmos.

In the first three months of running the biggest, most sensitive dark matter detector yet — a vat of 368 kilograms of liquid xenon cooled to minus 150 degrees Fahrenheit — the researchers said they had not seen a trace of the clouds of particles that theorists say should be wafting through space, the galaxy, the Earth and, of course, ourselves, knocking out at least one controversial class of dark matter candidates.

But the experiment has just begun and will run for all of next year. The detector, already twice as sensitive as the next best one, will gain another factor of sensitivity in the coming run.

“Just because we don’t see anything in the first run doesn’t mean we won’t see anything in the second,” said Richard Gaitskell, a professor of physics at Brown University and a spokesman for an international collaboration that operates the experiment known as LUX, for the Large Underground Xenon dark matter experiment.

As has become de rigueur for such occasions, the scientists took pride and hope in how clearly they did not see anything. “In 25 years of searching, this is the cleanest signal I’ve ever seen,” Gaitskell said in an interview.

That meant, the scientists said, that their detector was working so well that they would easily see a dark matter particle if and when it decided to drop by.

In this case, they had support from outside scientists. Neal Weiner, a particle theorist at New York University, called the results impressive.

“They have not found dark matter,” he said. “There is nothing smacking you in the face to make you think there is something there.” But as the sensitivity of the detector increases, he added, “If there is anything in there, it should become apparent.”

The announcement at the Homestake site capped a morning of ceremony, which included Governor Dennis Daugaard of South Dakota and members of the state Legislature, at what amounted to a coming-out party for LUX and for the Sanford Underground Research Facility, a lab being developed in the old mine with a mix of state and private money, as well as support from the Energy Department. The lab is named after the philanthropist T. Denny Sanford, who donated $70 million to get it going.

LUX is the latest in a long series of ever-larger experiments that have occupied and taunted the world’s physicists during the last few years. They are all in abandoned mines or other underground sites to shield them from cosmic rays, which could cause false alarms. Daniel McKinsey, an associate professor of physics at Yale and a spokesman for the LUX group, said in an interview that the biggest source of noise in the LUX device was trace radioactivity in the detector itself.

Larger instruments are on the drawing boards of LUX and other collaborations, but physicists say the experiments are already sensitive enough to test some versions of dark matter that have been proposed, including the idea that dark particles interact with ordinary matter by exchanging the recently discovered Higgs boson. Weiner said he held his breath every time new results from a dark matter experiment were released.

Dark matter has teased and tantalized physicists since the 1970s, when it was demonstrated that some invisible material must be providing the gravitational glue to hold galaxies together. Determining what it is would provide insight into particles and forces not described by the Standard Model that now rules physics, not to mention a slew of Nobel Prizes.

Physicists’ best guess is that this dark matter consists of clouds of exotic subatomic particles left from the Big Bang and known generically as WIMPs, for weakly interacting massive particles, which would weigh several hundred times as much as a proton but could nevertheless pass through the Earth like smoke through a screen door. They are a generic feature of a much-hyped idea known as supersymmetry.

Particle physicists have been hoping to produce these particles or other evidence of supersymmetry in the Large Hadron Collider outside Geneva or to read their signature in cosmic rays from outer space. No one has ever claimed to have seen such a heavy WIMP, in space or underground, but another experiment in another mine, the Cryogenic Dark Matter Search, claims to have recorded three events that could have been low-mass dark matter particles, only a few times heavier than a proton.

The new results from the Homestake mine, if they are correct, would rule out those low-mass particles. Gaitskell explained that if those particles were real, the Homestake detector would have recorded 1,550 of them.

“If there are 1,550 of them, boy are we going to see them,” he said in a presentation at the Homestake facility on Wednesday. “We do not see the low-mass WIMPs.”

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