Disputed dark-matter claim to be tested by new lab in South Korea

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Disputed dark-matter claim to be tested by new lab in South Korea


It’s a mystery that has had physicists scratching their heads for more than 20 years. The DAMA/LIBRA experiment at the Gran Sasso National Laboratory (LNGS) near L’Aquila, Italy, has been recording an annual fluctuation of light flashes in its detector that appears to be a sign of dark matter. But no one has been able to definitively replicate the findings.

But beneath a mountain in Jeongseon, South Korea, researchers are scaling up an experiment that could finally lay the controversial dark-matter claim to rest. In June, researchers will finish installing a revamped detector in a brand-new facility called Yemilab. If all goes to plan, the upgraded COSINE-100 experiment will be running by August, says Hyun Su Lee, a physicist at the Institute for Basic Science (IBS) in Daejeon, South Korea.

Dark matter is thought to account for 85% of mass in the Universe, but because it barely interacts with ordinary matter and doesn’t interact at all with light, it is notoriously difficult to observe directly. Several research teams have tried to catch a glimpse of the elusive substance, but only the DAMA/LIBRA experiment has claimed to have seen it for real.

The prospect of confirming the observation of dark matter has captured physicists’ attention. “There is a big effort in the dark-matter community to reproduce this result,” says Nicola Rossi, an experimental particle physicist at LNGS.

Apples and apples

DAMA/LIBRA’s observations of the distinct annual pattern is consistent with what physicists would expect with Earth’s relative position in the galaxy throughout the year. As the Earth orbits the Sun, the Sun orbits the black hole at the centre of the Milky Way. In June, the Earth hurtles through the Milky Way in the same direction as the Sun, increasing its relative speed through the haze of dark matter. But in December, the Earth travels with the flow of dark matter as it moves in the opposite direction to the Sun. As expected, the number of signals recorded by DAMA/LIBRA’s detector are highest in June and lowest in December.

A handful of groups have attempted to reproduce DAMA/LIBRA’s results using similar methods and materials in their detectors, including the same type of sodium iodide crystals that emit tiny flashes of light when they are hit with subatomic particles. Among them is COSINE-100, which has been running since 2016 at Yemilab’s predecessor — the Yangyang Underground Laboratory (Y2L) in South Korea. But none have produced results that match those of the original experiment, raising questions about whether the yearly swing in signals is because of something else, such as the detector itself or from errors in the analysis methods used. “This is a puzzle that’s still there after 20 years,” says María Luisa Sarsa, a physicist who works on the ANAIS-112 experiment, which is also focused on replicating DAMA/LIBRA’s results, at the Canfranc Underground Laboratory in Huesca, Spain.

The access tunnel to Yemilab. It is accessible by pedestrian elevator, and by vehicle through a tunnel.Credit: Kangsoon Park and Eunkyung Lee

But to confirm or rule out DAMA/LIBRA’s claims for good, experiments need to match the original as much as possible, says Henry Tsz-King Wong, a physicist at Academia Sinica in Nangang, Taiwan. Although the COSINE-100 detector uses the same type of sodium iodide crystals, they contain up to three times more radiation than the ones used in DAMA/LIBRA, which can muddy the faint signals of potential dark-matter particles and make it difficult to produce definitive results. “The community would very much like to have something that is really comparable, apples-to-apples identical,” says Wong, who works on the Chinese Dark Matter Experiment at the China Jinping Underground Laboratory in Sichuan.

The upgraded experiment will use the same crystals as those used in the earlier COSINE-100 experiment, but with some extra enhancements to boost their sensitivity, says Lee. The team are also developing a set of sodium iodide crystals that will be even more radiopure than DAMA/LIBRA’s for the next phase of the experiment, COSINE-200. With even lower radioactivity levels, the hope is to generate enough data over a shorter period of time to reach a more solid conclusion about DAMA/LIBRA’s results, and also search for low-mass dark matter, says Lee, who co-leads COSINE-100. “A lower background detector would simplify all aspects of analysis,” he says.

Neutrinos

COSINE-100 will be housed in the brand new 3,000-square-metre Yemilab. The 31 billion won (US$23 million) facility lies around 1 kilometre underground and surpasses Y2L in both depth and volume. Since September 2023, researchers have been moving all Y2L experiments to Yemilab, where they will begin their next phase by the end of this year.

A view of the cavernous AMoRE experimental hall.

The AMoRE hall of Yemilab, which will search for neutrinoless double β decay.Credit: Kangsoon Park and Eunkyung Lee

Yemilab also offers a better-shielded environment for detecting elusive particles besides dark matter. The facility will also hunt for neutrinos — chargeless particles that barely have mass. The second phase of an experiment called AMoRE will search for signs of two neutrons decaying into protons and electrons without emitting a neutrino. This hypothesized process is called neutrinoless double β decay and if observed, it will demonstrate that neutrinos are their own antiparticle. This could offer clues about their mass and explain why there is more matter than antimatter in the Universe, says Yeong-duk Kim, a physicist at IBS and AMoRE spokesperson. The upgraded neutrino detector will use around 160 kilograms of crystals embedded with molybdenum-100, a naturally occurring radioisotope. When AMoRE-II starts running at the end of this year, it will be 100 times more sensitive than the previous version of the experiment, adds Kim.

Whether the two experiments succeed or fail at detecting the rare events they are looking for, they are nevertheless set to raise more questions, says Rossi. “If both will deliver only null results, we should seriously start rethinking the Universe,” he says.



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