Wednesday, May 30, marked the official opening of the Davis Campus of the Sanford Underground Research Facility, 4,850 feet down in the former Homestake gold mine in Lead, South Dakota. To carry more than 60 VIPs from the state and the U.S. Department of Energy, scientists from a slew of universities and national laboratories, and local and national media required four trips in an open elevator cage, making the 10-minute descent to the nearly mile-deep lab. It was a trip back in time—and then abruptly forward again.
The Homestake mine opened in the 19th century, and much of the technology of its immense spinning hoists and its shafts framed with water-dripping wooden beams still smacks of the 1930s. But when the cage door rolls open, it takes a visitor only a few steps to reach a modern laboratory facility that could be anywhere.
The floors are steel grids and smooth painted concrete, the ceilings are acoustic tile or overhead conduits packaged in shiny foil, and the walls are ho-hum cinder block—except when they suddenly bulge into Gaudiesque undulating freeforms, the shotcrete natural rock face intruding into the laboratory space.
Deep underground is the only place to do the kind of leading-edge physics experiments that will be underway in these laboratories within the next weeks and months. LUX is a search for dark matter, and the MAJORANA DEMONSTRATOR is the first step in an exhaustive search to show that neutrinos are their own antiparticles, by observing something called neutrinoless double-beta decay. Both require the absolute minimum of background interference.
“The 4,850 feet of rock screens out most cosmic rays, and the surrounding rock is lower in radioactivity by a factor of 10 or more than other underground locations, including even those deeper than the Davis Campus,” says Kevin Lesko of Lawrence Berkeley National Laboratory’s Nuclear Science Division. Lesko is principal investigator for the Sanford Lab, whose operations are funded by DOE through Berkeley Lab.
MAJORANA DEMONSTRATOR’s goal is to prove that background noise at the Davis Campus is indeed “quiet” enough to be worth the expense of searching here for neutrinoless double-beta decay, a process with an estimated half-life longer than a trillion times the age of the universe (if it happens at all). LUX too, a search for weakly interacting massive particles (WIMPs), is not only the most sensitive search yet, it’s a precursor to a bigger detector to be placed in the same spot, if it’s quiet enough.
Quiet was the reason Ray Davis came from Brookhaven Lab in the mid-1960s, while Homestake was still a working gold mine, to look for neutrinos from the sun. He found only a third the number he expected, and the result was modern neutrino science, including the revelation that neutrinos have mass and come in flavors that oscillate among themselves. Three decades later he was recognized by a Nobel Prize.
Davis died just six years ago. His widow, Ana, was on hand with their son and daughter-in-law to help dedicate the Davis Campus, bringing along an official replica of his Nobel Prize medal (the original is in a bank vault). Sanford Lab officials presented her with a fragment of his experiment, which was torn down to build the water tank for LUX shielding that now stands in the same space.
The Campus’s official opening covered a wide spectrum of significance, with different meanings for different visitors. But for all parties, the day came down to science worth doing.
When asked what would happen if MAJORANA finds nothing at all, Associate Director for High Energy Physics in the DOE Office of Science James Siegrist said, “If there’s no observation, it tells us that neutrinoless double-beta decay doesn’t occur within well-defined limits, so neutrinos are not Majorana particles, they’re Dirac particles. That’s a step forward.”
And if LUX finds no WIMPs? Tom Shutt, the LUX co-PI from Case Western Reserve University, gave a straightforward answer. “We know dark matter interacts gravitationally and doesn’t interact via electromagnetism or the strong nuclear force. So if doesn’t interact weakly either, we start looking for a fifth force in the universe.”
“There’s a result no matter what,” Siegrist said. “DOE is going to benefit greatly from this data, with a good look at the future of physics.”