South Korean lab to hunt for dark matter after ambiguous Italian results

A new dark matter detector under a mountain in Jeongseon, South Korea, is about to be turned on. The team is trying to rule out – or perhaps reproduce – the strange results from an earlier detector in Italy.

As far as astronomers who study the observable universe can tell, only about 5 percent of it is made up of matter. The rest, or most of it, consists of dark matter (about 27 percent) and dark energy (about 68 percent).

Dark matter is invisible matter that does not emit its own light and only interacts with normal matter through gravity, which we can see evidence of in galaxies and galaxy clusters. But given that there appears to be five times more than ordinary matter, scientists are certainly looking for direct evidence of its existence.

One approach to finding it, perhaps in reverse, since dark matter explains what we see in stars and galaxies, is to go underground.

“On Earth,” explained Hugh Lippincott, associate professor of physics at the University of California, Santa Barbara, in an article for The Conversation, “we are constantly surrounded by low, non-hazardous levels of radioactivity from trace elements – mainly uranium and thorium – in the environment, as well as cosmic rays from space”.

“The goal in dark matter hunting is to build a detector as sensitive as possible so that it can see dark matter and place it in as quiet a place as possible so that the signal dark matter can be seen above the background radioactivity.”

There are several underground facilities around the world where physicists look for signs of Weakly Interacting Massive Particles (WIMPs), among other things, such as measuring the impact of neutrinos. The idea is that WIMPs must pass through the Earth all the time as it moves through space, so to detect them we simply need detectors sensitive enough to pick up those weak interactions.

So far, basically, no luck. But in 1997, a team in Italy – the DAMA/LIBRA experiment at the Gran Sasso National Laboratory – reported strange results. Their detector uses sodium-iodide crystals, which produce tiny flashes of light when a subatomic particle interacts with a nucleus inside these crystals. The team reported flashes of light, signs that they had been hit by subatomic particles. This is to be expected, with some of the background events mentioned above making their way to the detectors.

However, the number of events varied throughout the year, which is what you would expect if some of the events were caused by dark matter. This is because Earth moves through the Milky Way’s dark matter halo faster during certain parts of its orbit, increasing the number of detections you would expect. As it moves more slowly through the halo, you would expect detections to be lower as well.

The results were controversial, especially as other teams have tried and failed to replicate the results. But the team continued to see these discoveries even as they increased the sensitivity of their instruments. Muddying the waters still, as the detector’s sensitivity increased, it should have been able to detect lower-energy collisions. These would peak at different times of the year, but this was not seen in the data.

Now, a team in South Korea is trying to replicate the DAMA/LIBRA experiment in a new facility called Yemilab. The team’s previous experiment – COSINE-100 – was unable to replicate the DAMA/LIBRA results, despite using a similar structure using the same sodium-iodide crystals.

“Initial results reveal a fair portion of the potential dark matter search region attracted by the DAMA signal,” said Hyun Su Lee, co-spokesperson for COSINE-100 in a 2016 statement. “In other words, there is little room left. because this claim is from the dark matter interaction, unless the dark matter model is modified significantly.”

The new experiment – called COSINE-200 – uses the same setup, but with increased sensitivity and more “radiopure” sodium iodide crystals. Overall, the project – which should start from August – is also more shielded than the DAMA/LIBRA experiment, hoping to reduce any background interference that may have occurred in earlier experiments.

Hopefully, the data gathered will tell us whether the previously claimed discovery was a mistake, other sources (always a possibility, just ask the team that thought they saw faster than light neutrinos), or something really weird.