Yesterday's rivals, today's colleagues. This is the story of Italy's Gran Sasso National Laboratory (LNGS), which operates the world's largest underground experimental facility and has produced major results in dark matter research, and Korea's Institute for Basic Science (IBS). The two institutions, which had been at odds for years over dark matter, the mystery of the universe, became one team on the 30th. They set up a joint research center to search for dark matter together.
Aldo Ianni, LNGS head of international cooperation, said at the opening ceremony of the joint research center (GCND), "IBS's research results are very solid, and I agree with them," and noted, "Cooperation between the two sides is needed to identify the origin of the annual modulation signal claimed by our dark matter experiment and to advance dark matter research." Kim Young-deok, IBS Director General of the Center for Underground Physics, explained, "The fact that the two research institutions, which have produced conflicting results, created a joint center means we will end the controversy and develop new dark matter detection technologies."
◇ Korea rebuts its competitor with experiments using the same device
Scientists say that only 5% of the universe consists of matter that emits or reflects light like stars or planets and can be observed by us, 68% is dark energy that pushes the universe to expand, and 27% is dark matter that does not emit light and pulls objects. What dark matter is and how it works has yet to be revealed.
The IBS Center for Underground Physics has been searching for dark matter at Yemi Lab, Korea's only deep underground research facility located 1,000 meters beneath Jeongseon, Gangwon Province. The team recently drew international attention by directly refuting DAMA, a flagship experiment at LNGS. The results were published in September in the international journal Science Advances.
The focus is on WIMP (weakly interacting massive particle), identified as a dark matter candidate. It is a particle much heavier than an electron but that barely interacts with other matter, making it hard to capture. This is why researchers go deep underground. After blocking noise such as cosmic radiation with the mountain itself, they detect the faint scintillation produced when a WIMP nudges an atomic nucleus by converting it into an electrical signal.
As Earth orbits the sun, its relative velocity to dark matter in the galaxy changes, and the frequency of detected signals may therefore vary by season. In 1998, the DAMA team at LNGS observed such an annual modulation signal with a sodium iodide (NaI) detector and claimed it as evidence for the existence of WIMPs. But other teams failed to reproduce the result, fueling controversy over whether it was an experimental error.
COSINE-100, an international collaboration led by IBS, verified the DAMA claim. In 2016, the team conducted experiments at a 700-meter-deep underground lab in Yangyang, Gangwon Province, using the same type of detector as in Italy. An analysis of six years of data from 2016 to 2023, using the same method as the Italian team, found no annual modulation signal.
A Spanish team also conducted experiments in an underground laboratory using the same method and reached the same conclusion. This effectively ruled out the possibility that the signal captured by the Italian side was due to dark matter. The cross-analysis by IBS and the Spanish team has been accepted for publication in the international journal Physical Review Letters.
◇ Joint center: "Build together and verify together"
The research center jointly established by Korea and Italy will be operated with each country taking on specific roles. Korea will purify sodium iodide crystals to detect unknown dark matter signals, and Italy will handle crystal growth and radioactivity measurements.
Ianni said, "To find the rare signals of dark matter, the key is to block radioactive gases like radon and keep the detector clean," adding, "If Korea makes the materials as clean as possible, Italy will grow and validate them to produce the final samples for detection." Based on these samples, the teams will also develop next-generation detectors with higher sensitivity.
Ianni said, "Korea's Yemi Lab (1 km underground) and Italy's LNGS (1.4 km) differ in depth, allowing comparisons of radiation environments," and added, "Research on underground biological responses helps us understand the environment for rare particle experiments and, further, informs the design of space experiments."
With no definitive results from WIMPs, researchers are broadening their scope. Lower-mass dark matter, or axions and axion-like particles (ALP), have emerged as new candidates. Because they do not leave traces by colliding with atomic nuclei like WIMPs, indirect detection using magnetic fields, electromagnetic fields, and resonance phenomena is needed.
Ianni said, "There is no guarantee that dark matter is a single particle," and added, "It could be a mixture of several types, like ordinary matter." He noted, "Narrowing the candidate space by combining astronomical observations is important for setting research directions," and said, "Within the next 10 years, we will figure out which detection method is most effective."