Dark-matter search gets started deep in Sanford Lab

The outlook for building a US underground laboratory nosedived in late 2010 when NSF pulled out of the project. But quietly and with little fanfare, such a lab is being realized in the defunct Homestake gold mine, thanks to unwavering support from the host state of South Dakota and philanthropist T. Denny Sanford and to the Department of Energy. In August 2011 DOE stepped up with operations money to keep alive two experiments in the mine: the Large Underground Xenon (LUX) search for dark matter and the Majorana Demonstrator for neutrinoless double beta decay (see Physics Today, February 2011, page 21 , and August 2011, page 23 ).

“DOE had thought of itself as putting experiments in the facility,” says Los Alamos National Laboratory’s Steven Elliott, spokesman for the Majorana Demonstrator. “Taking responsibility for the facility itself was different.” Tensions were high for a while, he says. “But the longer we are there, and DOE supports us, the more confident we become. We hope we are not the turkey before Thanksgiving.” DOE is so far ponying up $15 million a year; its activities at the lab are overseen by a team based at Lawrence Berkeley National Laboratory.

Now known as the Sanford Underground Research Facility, the site has been excavated, renovated, and outfitted using some of the $70 million from Sanford and $40 million from South Dakota, and the governor has requested another $2 million in state funding for the next fiscal year. The Sanford Lab celebrated its opening on 30 May 2012. The next day, experiments began moving in.

Both the Majorana Demonstrator and LUX were poised to begin collecting data as Physics Today went to press. And on 10 December 2012, the DOE granted preliminary approval to the Long-Baseline Neutrino Experiment (LBNE), in which intense beams of neutrinos would be shot 1300 km from Fermilab to the Sanford Lab. “That is a very big deal,” says LBNE cospokesman Bob Svoboda of the University of California, Davis. “Ongoing projects weather things like continuing resolutions better than new [projects].” Moreover, the prospect of the Sanford Lab hosting the LBNE, with its 20-year lifetime, could well attract to the site other experiments that require shielding from cosmic rays.

Getting started

The LUX detector is located 4850 feet deep in the Davis campus—named for Ray Davis Jr, whose pioneering solar neutrino experiment began running in the Homestake mine four decades ago. The dark-matter detector consists of 350 kg of xenon in a cryostat suspended in a 70 000-gallon tank of purified water. Experimenters are looking for a recoil signal—of both light and charge—when weakly interacting massive particles pass through. The WIMPs would have a characteristic energy deposition of about 10 keV. “After a few weeks, we will have surpassed the combined sensitivity of all other dark-matter experiments,” says cospokesman Daniel McKinsey of Yale University. “We have a bigger detector, and the detector itself has low radioactivity.” Whether or not dark matter is spotted, the plan is to run LUX through early 2015, with hopes to then scale up to a larger xenon detector, dubbed LZ.

The Majorana Demonstrator is testing its first detectors. Germanium-76 is both the source and detector for neutrinoless double beta decay, and the experiment hinges on reducing the background (see Physics Today, January 2010, page 20 ). To reduce radiation from the apparatus itself, the copper cryostats and other components are being electroformed underground in a clean room at the Sanford Lab. For increased sensitivity, three-quarters of the 40 kg of germanium is being enriched at $90 per gram from its natural abundance of 7.5% to 86% ⁷⁶Ge. “We will either see or place a limit on the rate of double beta decay,” says Elliott. “But what DOE has mandated us to do is to measure the background in the relevant energy region”—a 4-keV spread around 2039 keV.

That, he says, will take about three years. “Then we will compare with the GERDA experiment being built in Europe [in Italy’s Gran Sasso National Laboratory] on a similar time scale,” Elliott says. “We hope to compare background rates, and then for the two collaborations to come together and cherry-pick technologies that work best and propose a larger, ton-scale double beta decay experiment.”

Determining whether neutrinos are Majorana—that is, their own antiparticles—is likely to require a larger detector. But even with 40 kg of ⁷⁶Ge, scientists expect to test the controversial Heidelberg–Moscow claim in which a subset of scientists in an experimental collaboration claims to have seen lepton number violation.

The Sanford Lab site is also host to a smattering of other experiments in geology, physics, engineering, and biology. Instruments are located at multiple levels, says lab spokesman Bill Harlan. “Mostly they are small experiments and don’t require a lot of infrastructure.”

Another potential tenant is DIANA, the Dual Ion Accelerators for Nuclear Astrophysics. Two underground accelerators covering the energy range 50 keV to 3 MeV would measure various nuclear reactions that occur in the Sun and stars. “The reactions have extremely low cross sections. That is why stars live so long,” says project principal investigator Michael Wiescher of the University of Notre Dame. The roughly $50 million DIANA does not yet have funding. The Sanford Lab is one of three possible sites, along with an active lime mine in Kimballton, Virginia, and the Soudan mine in Minnesota.

Bare-bones long baseline

The version of the LBNE now on the table is a far cry from what scientists are actually hoping to build. In slashing the price tag from about $1.5 billion to $867 million as DOE required, says Svoboda, “we went through the budget with a fine-toothed comb. Electronic security system? We put a padlock.”

The facility would be central to Fermilab’s post-Tevatron strategy to develop a world-leading program in the intensity frontier of particle physics. In its full glory, the LBNE would have a 34-kiloton underground detector in South Dakota and another detector close to Fermilab to keep tabs on what is actually in the outgoing beam. But to almost halve the price, drastic measures are proposed: Instead of a near detector for neutrinos, the reduced version would have a simpler muon detector, which gives indirect information about what is in the neutrino beam. The Sanford detector would be trimmed from 34 tons to 10 tons, and instead of going underground, it would sit on the surface at the South Dakota site.

“What we did not compromise is the long-distance, upgradeable neutrino beam” at Fermilab, says Svoboda. “You can’t add that back.” In the bare-bones version, neutrino oscillations could be studied because the incoming neutrinos arrive in pulses and can be distinguished from the background. But a full-sized underground LBNE would have much broader applications. It could be used to look for supernovae, atmospheric neutrinos, and proton decay, among other things. “This is our frustration,” says Jim Strait, LBNE project director. “You could get a lot more science for an incremental amount of money.” Putting the detector underground would add about $130 million.

The LBNE team’s plan is to seek non-DOE and international partners to build a near detector, increase the far detector size, and put the far detector underground. “We are beating the bushes to find other resources,” Strait says. For starters, scientists in India have submitted a proposal to their funding agencies to build a near detector, which would help the LBNE oscillation physics studies and be used to make precision measurements of neutrino cross sections and electroweak parameters and to search for new physics.