The COSINE-100 detector was built to reproduce a controversial claim of the detection of dark matter.

Jay Hyun Jo

Underground experiment casts doubt on controversial dark matter claim

Twenty years ago, physicists with the Dark Matter (DAMA) experiment in Italy’s subterranean Gran Sasso National Laboratory announced they had detected particles of dark matter—the mysterious stuff whose gravity presumably holds the galaxy together. Now, the first experiment designed to directly test DAMA’s controversial claim has released its first data. Physicists working with the COSINE-100 detector in South Korea say they see no sign of dark matter—but still need a couple more years to really put the screws to the DAMA claim.

“They can’t rule out the DAMA signal yet,” says Katherine Freese, a theoretical physicist at the University of Michigan in Ann Arbor who is not involved in either experiment. “But the exciting thing is that they’ll be able to rule it out.” Or, as may be less likely, confirm it.

Astrophysical observations show invisible dark matter makes up 85% of all matter. Our own galaxy is thought to reside within a vast cloud of the stuff. However, scientists still don’t know what dark matter is. For decades, experimenters have hunted for particles of it floating about, mostly to no avail. To search for dark matter, physicists deploy ultrasensitive detectors deep underground, where they are shielded from cosmic rays and other background radiation.

However, since 1998, the DAMA collaboration has claimed to have seen just such a signal. The team’s detectors consist of sodium iodide crystals doped with thallium, which produce flashes of light when a particle of some sort—regular or dark matter—strikes a nucleus within the material and sends it flying. The DAMA team has seen a yearly variation in the collision rate that could be a strong sign of dark matter, as Freese and a colleague predicted in 1986.

If our Milky Way galaxy is shrouded in dark matter, then as the sun wheels about the galactic center, it should regularly plow into a wind of dark matter particles. Moreover, as Earth circles the sun, it should alternately rush into and out of that wind, causing the rate of dark matter collisions to wax and wane over the course of the year. If dark matter consists of theorists’ favorite candidate particle, known as weakly interacting massive particles (WIMPs), the signal should peak in June and bottom out in December—just what DAMA sees.

Several other detectors have failed to see the signal. However, those detectors use heavier elements such as xenon, silicon, and germanium for the target nuclei, DAMA researchers say, which could explain the discrepancy. “Even taking those results as they are, considering the large experimental and theoretical uncertainties there could be space for compatibility,” says Rita Bernabei, a physicist at the University of Rome Tor Vergata and leader of the DAMA team.

To sort through the confusion, COSINE researchers built a detector that also uses thallium-doped sodium iodide crystals. “I got into this field to test the DAMA result, and I was surprised others hadn’t,” says Reina Maruyama, a physicist at Yale University and co-spokesperson for the 50-member COSINE team. Since 2016, the 106-kilogram detector has been collecting data 700 meters underground at Yangyang Underground Laboratory, on South Korea’s eastern coast. And its first 59.5 days of data show no sign of dark matter, COSINE researchers report today in Nature.

So does the COSINE result nix the DAMA claim? Not quite. With only 2 months of data, COSINE researchers couldn’t look for the telltale annual variation, but simply looked for an excess of events above the backgrounds created by extraneous radiation. The lack of an excess rules out the possibility that DAMA is seeing WIMPs, Maruyama says. But Bernabei says the test is too weak to do that. “The modeling of a background is a quite uncertain procedure and at low energy is in general not reliable,” she says.

However, Freese says the simplest version of WIMPs are already ruled out—by DAMA’s own data. The argument is tricky, but the simplest version of WIMPs are supposed to interact with the nucleus in a particularly simple way that does not depend on the nucleus’s spin. And in that case, the peaks and valleys in DAMA’s annual cycle should shift by 6 months for lower-energy events, Freese explains. However, low-energy data that DAMA presented earlier this year show an unshifted oscillation. DAMA could be seeing some more complex version of WIMPs or some other kind of dark matter particle, Freese says. Bernabei argues that DAMA could still be seeing the simplest version of WIMPs.

All agree that to really put the DAMA claim to the test, COSINE researchers will have to look for the same annual variations that DAMA sees—which can help pull a weaker signal out of the background. COSINE already has 2 years of data in the can, Maruyama says, and it will need another 3 years to make that test. Two other experiments are also trying to directly challenge the DAMA result with sodium-iodide detectors.

Ultimately, all physicists hope to detect dark matter. So Maruyama says she would “love to” reproduce the DAMA signal. If COSINE cannot do that, Freese says, “We may never know what created the DAMA signal.”

*Correction, 7 December, 11:27 a.m.: This story has been corrected to note that, according to Freese, the DAMA results are inconsistent with only the simplest version of WIMPs, and not all versions of the hypothetical particle as previously implied.