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The LZ experiment in a South Dakota mine will use a 7-tonne tank of liquid xenon to look for WIMPs.

The LZ experiment in a South Dakota mine will use a 7-tonne tank of liquid xenon to look for WIMPs.

Matt Hoff, Berkeley Lab

Two big dark matter experiments gain U.S. support

For a change, U.S. particle physicists are savoring some good news about government funding. The Department of Energy (DOE) and the National Science Foundation (NSF) announced on Friday that they will try to fund two major experiments to detect particles of the mysterious dark matter whose gravity binds the galaxies instead of just one. The decision allays fears that the funding agencies could afford only one experiment to continue the search for so-called weakly interacting massive particles, or WIMPs. It also averts having to choose between the two leading WIMP-search teams in the United States.

"We have the opportunity right now for the U.S. experiments to push further in sensitivity and possibly make a discovery," says Richard Gaitskell, a physicist at Brown University and a member of the team developing a WIMP detector called LZ, one of the two leading projects. "There's a real commitment from the community and the funding agencies." Blas Cabrera, a physicist at Stanford University in Palo Alto, California, and spokesman for the rival SuperCDMS experiment, says that having to pick only one team “would have been a grave mistake."

For decades, astronomers and astrophysicists have reasoned that some sort of otherwise unobservable dark matter provides most of the gravity that keeps the galaxies from flying apart. Physicists hope to identify that stuff by detecting particles of it floating around us. For example, dark matter could consist of WIMPs, hypothetical particles that would barely interact with ordinary matter and weigh much more than protons.

The current generation of experiments aims to detect WIMPs as they crash into atomic nuclei in ultrasensitive detectors deep underground, where they're shielded from ordinary radiation. The next generation, to run later in the decade, would be bigger, more sensitive, and more costly. But in 2012, DOE officials estimated that they had only $29 million to spend on second-generation dark matter searches—roughly the amount that either of the two leading teams needed for its planned upgrade.

LZ would be a much bigger version of an experiment called the Large Underground Xenon dark matter experiment, or LUX, now running 1480 meters down in the Sanford Underground Research Facility in Lead, South Dakota. It would consist of a tank of 7 tonnes of frigid liquid xenon, which would emit telltale flashes of light when a WIMP strikes a nucleus. In contrast, SuperCDMS would be a larger version of the Cryogenic Dark Matter Search II (CDMS-II), which used hockey puck–sized disks of solid germanium to detect WIMPs through heat and electrical signals and ran in the Soudan Underground Laboratory in Minnesota. SuperCDMS will run in the deeper SNOLAB in Sudbury, Canada.

LZ will cost about $55 million, although roughly $20 million would come from foreign partners and the South Dakota Science and Technology Authority. Harry Nelson, a physicist at the University of California (UC), Santa Barbara, and spokesman for the LZ team, says “my jaw hit the floor” when officials from both agencies told him that “they intend to fully fund us.”

In contrast, researchers with SuperCDMS didn't get everything they had wanted. Physicists had hoped to build a detector capable of containing 400 kilograms of germanium and outfitted with an initial 110 kilograms for a cost of $32 million, $3 million of which would come from the government of Canada. Instead, they will start with a full-sized rig, but only 50 kilograms of germanium. And whereas LZ will continue to search for WIMPs weighing several hundred times as much as a proton—as many theorists expect they should—SuperCDMS researchers will narrow their focus to much lighter WIMPs only a few times the weight of a proton. Nevertheless, Cabrera is upbeat. "I think we need a broad approach, and I am perfectly happy to embrace this low-mass region," he says.

In an article in the DOE particle physics publication Symmetry, the two agencies said they will also fund the relatively inexpensive Axion Dark Matter Experiment (ADMX) at the University of Washington, Seattle. ADMX stalks a different type of dark matter particle entirely, the axion, which would be far lighter than WIMPs and could be converted into a photon in a magnetic field.

The news was not as good for two other WIMP searches, however. DOE and NSF declined to fund further development of PICO, an experiment at SNOLAB that uses a device called a bubble chamber to track nuclei recoiling from a WIMP collision, and DarkSide, a detector in Italy's subterranean Gran Sasso National Laboratory that is filled with liquid argon. Researchers who spoke to ScienceInsider say they expect the two agencies to continue funding research on those technologies, however.

Federal officials are not saying where the extra money will come from, although a recently released road map for U.S. particle physics by the community calls for increased funding for dark matter searches. UC Santa Barbara's Nelson says he believes the agencies plan to roughly double their previous investment of $29 million for the next generation of dark matter searches. However, James Whitmore, an NSF program director for particle physics, declined to provide any funding details, and James Siegrist, DOE's associate director for high-energy physics, did not respond to a request for comment.

Researchers also say they're not counting their chickens just yet. They are worried that the green light will turn yellow if Congress extends current funding levels for all agencies because of its failure to pass a 2015 budget by the start of the fiscal year on 1 October.

*Correction, 21 July, 2:34 p.m.: The story has been changed to more accurately explain the original SuperCDMS proposal.