Dark-matter intrigue

At the April 2013 meeting of the American Physical Society (APS) in Denver, researchers with the Cryogenic Dark Matter Search (CDMS) announced the detection of three particles that maybe, possibly, could have been dark matter. Despite skepticism from attendees and even the CDMS scientists, some physicists couldn’t help but get excited. In the decades-long quest to produce, remotely sense, or directly detect the universe’s most abundant yet elusive variety of matter, notable experimental discoveries have been few and far between.

Which is why at the same meeting I couldn’t help but ask attendees about DAMA, the dark-matter experiment in Italy that has consistently detected something—perhaps dark matter, perhaps not—since the late 1990s. Here were scientists cheering three flashes of light in a silicon-filled cavern, yet a longstanding result with far stronger statistical significance was largely ignored.

A technician examines the DAMA experiment’s detectors. Credit: DAMA/LIBRA Collaboration

So why isn’t DAMA taken more seriously? I set out to answer that question (and some others) now that experiments are finally coming online to put DAMA to the test. My article appears in the July issue of Physics Today.

Digging into DAMA reveals more than just scientific discrepancies. In an era that increasingly favors open sharing of data, DAMA keeps things close to the vest. After contracting with a crystal-growing company to produce ultrapure sodium iodide for the experiment, DAMA requested that the company not share the technology with other researchers (the request has since been rescinded). Only now have other scientists figured out how to grow crystals that come close to rivaling the purity of DAMA’s.

From the perspective of much of the dark-matter community, DAMA researchers have chosen to alienate themselves. In return, DAMA appears distrustful of other scientists and the media. The excerpt from a Rudyard Kipling poem that appears on the group’s home page is telling: “If you can bear to hear the truth you’ve spoken twisted by knaves to make a trap for fools, … you’ll be a Man my son!”

Of course, just because scientists aren’t as open and collaborative as others would like doesn’t mean they’re wrong. Physicists have had nearly two decades to explain away DAMA’s claimed dark-matter signal and have been unable to do so. The detectors and results are clearly impressive enough to motivate several new experiments: DM-Ice , ANAIS , and SABRE .

Frank Calaprice , the Princeton University physicist leading SABRE, was contemplative on the subject at the April 2016 APS meeting in Salt Lake City. We had just come out of a session dedicated to the life of astrophysicist John Bahcall , who embraced the unexpected results of the neutrino-hunting Homestake experiment and pursued the solar neutrino problem for decades despite ridicule from some colleagues. Calaprice couldn’t help but notice a few parallels with DAMA.

“It’s part of the reason I got into this,” Calaprice says. “The data look pretty good, everyone says it’s wrong—I’ve heard this before.”