Some 2,400 metres below the Jinping Mountains in southwest China, the world’s deepest and largest underground laboratory has just opened. The enormous space is home to scientists who are hunting down dark matter — the hypothetical substance that is thought to make up more than 80% of the mass in the Universe.
The China Jinping Underground Laboratory (CJPL) opened in 2010 and, after three years of construction, its second phase, CJPL-II, became operational in December 2023. With a sprawling capacity of 330,000 cubic metres, it now surpasses the Gran Sasso National Laboratory (LNGS) in L’aquila, Italy, the previous record-holder for both depth and volume.
The extra space has allowed experiments such as the Particle and Astrophysical Xenon Experiments (PandaX) and the China Dark Matter Experiment (CDEX) to upgrade. “It’s amazing what they’ve been able to do in a decade,” says Juan Collar, a physicist at the University of Chicago in Illinois.
Dark matter remains a scientific mystery. Physicists have calculated that the gravity generated by visible matter is too weak to keep fast-moving galaxies from flying apart. So, they theorized dark matter as the invisible glue holding the Universe together. Although dark matter should be everywhere, it’s proven difficult to directly observe, because it’s thought that it barely interacts with ordinary matter and doesn’t emit, reflect or absorb light. Claims of detecting dark matter have been plagued by suggestions that the experiments might have been confounded by other signals.
Scientific glory awaits those who first detect dark-matter, and this ongoing quest is one of the biggest efforts in particle physics, says Henry Tsz-King Wong, a physicist at Academia Sinica in Nangang, Taiwan, who works on CDEX.
Light under the mountain
The best place to look for dark matter is underground, because the layers of rock shield detectors from background ‘noise’, such as cosmic rays — high-energy particles that shower down on Earth from space — that can drown out potential dark-matter signals, says Marco Selvi, a physicist at the National Institute of Nuclear Physics in Bologna. Attempting to detect dark matter on Earth’s surface is “like trying to hear the tiny voice of a child inside a stadium where everybody’s shouting”, he says.
Underground, CJPL-II is exposed to cosmic rays at 0.000001% the rate of the Earth’s surface, making it one of the best-shielded underground labs in the world. Its walls are also coated in a 10-centimetre-thick protective shield made from a mix of rubber, concrete and other materials that block water and radioactive radon gas, which can seep in from the surrounding rock and disrupt dark-matter detection experiments.
The facility’s research teams are already making use of the extra space. While CJPL-II was being built, the PandaX team upgraded its detector from a capacity of 120 kilograms of liquid xenon to 4 tonnes. When a potential dark matter particle collides with a xenon atom, its energy should convert into flashes of light that can be detected by photosensors.
The detector is catching up to LNGS’s XENONnT experiment (8.6 tonnes) and the LUX-ZEPLIN Experiment (7 tonnes), at the Sanford Underground Research Facility in Lead, South Dakota.
The PandaX-4T detector sits inside a 900-cubic-metre tank of water to shield it even more from stray particles, says team member Ning Zhou, a physicist at Shanghai Jiao Tong University in China. “With better sensitivity, we can play with the detector and test the different types of interactions,” he says. The team’s ultimate goal is to build a xenon detector with a capacity of 40–50 tonnes, which would rival Europe’s DARWIN Experiment, which is aiming for 40 tonnes, says Zhou.
Meanwhile, the CDEX team has also deployed a germanium detector, which targets potential dark-matter particles with an even lower mass than the ones xenon experiments are looking for, says CDEX team member Qian Yue, a physicist at Tsinghua University in Beijing. The CDEX detector has been upgraded from a capacity of 1 kilogram to 10 kilograms of germanium, with plans to build a detector array that contains one tonne. If a dark-matter particle slammed into this detector, the interaction should produce charges, which would be converted into electrical signals. Yue hopes to invite more international collaborators to join CDEX — which already includes researchers from India and Turkey.
Although the search for dark matter is globally competitive, having several underground labs around the world run similar experiments allows researchers to compare their results, says Selvi. In 2022, the PandaX team was able to confirm results from LNGS’ XENON experiment — which found that a surprising signal detected by XENON in 2020 was from background noise rather than dark matter — by using a similar approach.
But, Collar thinks that dark-matter research would benefit from new approaches and ideas rather than beating the competition with bigger, more-sensitive versions of the same detectors. “There is enough replication already,” he says.
Over the next decade, the CJPL-II teams will continue to improve the sensitivity of their detectors, says Zhou. He also hopes the global dark-matter research community will share and combine CJPL-II data sets with their own. “We still have lots of stuff to do,” he says.
This article is reproduced with permission and was first published on January 22, 2024.