Where Is Dark Matter? Another Experiment Fails to Find a Signal

By Ryan F. Mandelbaum on at

A dark matter experiment in South Korea may soon confirm or deny an outstanding piece of dark matter evidence.

Observations of the universe reveal that it consists mostly of stuff scientists don’t understand. Regular matter seems to make up less than 5 percent of the universe, dark energy around 68 percent, and the rest is dark matter, mysterious stuff that has only been detected by its gravity. Recently, one experiment called DAMA/LIBRA in Italy claimed to have spotted evidence for this dark matter. A new experiment, COSINE-100, just released its own results – but it hasn’t spotted DAMA/LIBRA’s signal.

“What we can say is that spin-independent dark matter,” referring to a simpler theorised dark matter model, “is not causing the DAMA signal,” said Reina Maruyama, associate professor of physics at Yale, told Gizmodo.

Physicists at DAMA/LIBRA in Italy have reported an annual signal in the experiment’s sodium iodide crystal detectors, which would emit a teeny blip of light should a hypothetical dark matter particle pass by and give a nudge. The researchers interpret this as a varying quantity of dark matter, changing based on the Earth’s orientation with respect to the rest of the galaxy. COSINE-100 is another experiment containing 106 kilograms of sodium iodide crystals, located at the Yangyang Underground Laboratory in South Korea.

Scientists used COSINE-100 to take data from October 20, 2016 and December 19, 2016, and didn’t find any evidence of the dark matter DAMA/LIBRA supposedly spotted. Though other experiments have also failed to pick up the signal, COSINE-100 is the first made from the same sodium iodide, so its failure to corroborate the DAMA/LIBRA findings is an even more significant sign that the Italian experiment might not actually be sensing dark matter.

DAMA/LIBRA’s spokesperson, Rita Bernabei from the University of Rome Tor Vergata, told Gizmodo that COSINE-100's results don’t change anything. She didn’t think that COSINE-100 was as sensitive to passing dark matter interactions its scientists claimed it was, and that the signal could still appear if “a more correct, realistic, and reliable” model of the potential background sources of noise was used. But Dan Hooper, University of Chicago physicist, told Science News that the COSINE-100 results were “another nail in the coffin.”

This is just the first stages of work at the South Korean experiment, Maruyama told Gizmodo. The team only hunted for signals above background noise, and still needs to search for annual modulation like the yearly signal seen by DAMA/LIBRA . Other sodium iodide detectors will similarly attempt to verify the prior claims, as we’ve reported.

But if a second experiment made from the same material can’t verify the claims made by the first, it’s a sign that the strange signal DAMA/LIBRA detected might not be dark matter after all. [Nature]