Beneath the hills of southern China, some 170km (105 miles) west of Hong Kong, Chinese and European scientists are assembling what will become the world’s largest detector of neutrinos – tiny, ghostlike particles that pass through our bodies by the trillions every second. At 700 metres (3,000 feet) below ground, the US$300 million Jiangmen Underground Neutrino Observatory (Juno) is designed to measure the mass of different types of neutrinos and better understand their role in the formation of the universe. To catch neutrinos – which are trillions of times smaller than a grain of sand – scientists are building a plastic sphere 13 storeys tall that will be filled with 20,000 tonnes of a special liquid and soaked in 35,000 tonnes of pure water, according to He Wei, who is overseeing construction at the site. “We are now assembling a crucial structure of Juno that is a 41 metre [135 foot]-wide steel shell for mounting all the detection components on,” said He, from the Institute of High Energy Physics, Chinese Academy of Sciences. As the most abundant building blocks of the universe, neutrinos are everywhere but seldom interact with anything, making them extremely hard to catch and study. However, they can help answer some fundamental questions about the world we live in: how was matter distributed across the universe as we see it today? Why is the universe made of matter instead of antimatter? Why does the universe exist at all? After previously collaborating with China in the quest, the US has withdrawn almost entirely from the project, leaving Europe to become the new partner in Juno, a move that could see the Chinese and Europeans take the forefront in this research in years to come. First, however, there are unexpected challenges to overcome to complete the facility, including pandemic lockdowns, according to the scientists involved. China’s confidence rests on building on basic research, scientists say Working with He are nuclear researchers Paolo Lombardi and Michele Montuschi from the National Institute for Nuclear Physics in Italy. Over the past month they have been working with their Chinese colleagues to assemble and install two purification plants for Juno, which they developed with a company near Milan. “We need an extremely quiet and pure environment to detect neutrinos, and that’s why we go underground to stay away from cosmic rays and we purify the liquid to make it as clean as possible,” said Lombardi, who previously worked on an Italian solar neutrino experiment and specialised in purification plant design, development and operation. The installation was heavy work, involving a 200-tonne truck crane, the removal of some ceilings, and rope-secured operations at dizzying heights. They also worked late into the night and in hot and humid weather but things went smoothly, Lombardi said. “I have to say it’s not easy to come to China at this point, but we’ve received great support from Chinese companies and collaborators. We are not only colleagues but also friends,” he said. The neutrinos studied by Juno will be produced by two nearby nuclear power plants. More than 99 per cent of the particles will slip through the detection liquid without a trace, but occasionally some will interact with the liquid to trigger two flashes of light, less than a millisecond apart. The flashes will be recorded by light-detecting phototubes, 43,000 in total, to count as one signal. According to He, scientists expect to see around 60 signals every day once the experiment is up and running. One plant is above ground, standing 15 metres (49 feet) tall and will remove heavy metal elements such as Uranium-238 and Thorium-232 from the liquid. The other is 11 metres tall and placed underground to remove gaseous impurities like Radon, Argon-39 and Krypton-85. Since joining the Juno collaboration in 2014, Italy has now become its biggest international partner, with more than 80 researchers on board. The US-China neutrino hunt that opened a path to discovery Its in-kind contribution adds up to about €9 million (US$9.66 million), including the €6 million purification plants and €2.6 million worth of electronics. Italian scientists will also contribute expertise in data analysis, simulation and other work. Juno has nearly 700 members from 78 institutions around the world, with about 300 from outside China. Germany, France and Russia are also contributing equipment or electronics, and each has dozens of researchers involved. The project has also attracted scientists from other European countries, Asia, and South America. There are only a handful of participants from the US, China’s biggest partner in a previous multinational neutrino project which ran from 2003 to 2020 at Daya Bay near Shenzhen in southern China. The Daya Bay experiment collected enough data within its first two months of operation to open a new research chapter, with the discovery of a new kind of neutrino transformation. The results not only beat similar experiments in Asia and Europe, but also put China on the particle physics map by dramatically expanding its neutrino talent team from a few people to about 100. The Daya Bay collaboration was much smaller than Juno, with about 240 participants, but one third were from the US. “It’s really a shame that our American colleagues were not able to join Juno, which is designed as a follow-on mission to Daya Bay,” said Cao Jun, Juno’s deputy co-spokesperson who also works for the High Energy Physics Institute in Beijing. The downward spiral in China-US relations of recent years led to a de facto decoupling of the two countries in high energy physics. A 40-year tradition of mutual visiting delegations envisioned by the late Chinese leader Deng Xiaoping ended in 2018. Scientists trace distant origin of ghostly neutrino particles Another major reason for the US withdrawal from Juno was the decision to pursue its own ambitious project, the Deep Underground Neutrino Experiment (Dune) which will consist of a particle accelerator to produce neutrino beams, and two massive neutrino detectors, one in Illinois and the other in South Dakota. So far, Dune has lured more than 1,000 collaborators from over 200 institutions around the world. While its price tag has soared to beyond US$3 billion, the project has also suffered from construction-related setbacks according to a report last month by Scientific American . Juno is expected to be completed in 2024, making it the first next-generation neutrino experiment to become operational, ahead of Dune and Japan’s Hyper-Kamiokande, both of which are scheduled to start taking data in 2027-28. According to Cao, Juno will spend five to six years collecting a total of 100,000 signals to solve the neutrino mass ordering question, before moving on to a more challenging problem: is a neutrino its own antiparticle? Cao said Juno could easily be repurposed to run a neutrinoless double beta decay experiment to answer that question, with Chinese scientists already studying possible schemes in the hope of enhancing existing techniques to build the world’s most advanced double beta decay experiment in the 2030s. “We want to keep pushing back the frontiers of particle physics with lower costs and, hopefully, collaborators from all over the world,” he said. In the short term, China’s stringent Covid-19 policies may intimidate international collaborators who need to travel to the country for work, according to Lombardi. His trip to Juno was almost an odyssey, starting with unpredictable cancellations of flights, expensive tickets, and multiple PCR and blood tests mandated for boarding a plane to China. After landing in Xiamen, in neighbouring Fujian province, Lombardi was quarantined in a hotel room for 22 days, as required by local authorities. He continued to be tested during the quarantine, including on the day he was released. “After 22 days in a closed room, I could only get Covid from the people who came to test me, no? This was a scientific nonsense,” he said. Is life unique to Earth? China’s space telescope aims to find answers Lombardi and his colleagues are returning to Italy next Friday, but he says that if the present quarantine rules persist, it will be difficult for him to fly back into to China later this year for the commissioning of the Juno purification plants. And he warned that without a change in policy by the Chinese government, most of his European colleagues may choose not to come to China. “It’s such a pity because I like to come to China and I’m very happy to be part of such an exciting enterprise,” Lombardi said, but he still hopes that something will change by the end of the year, so he can work with his colleagues in Jiangmen once more.