American writer Dan Brown, in his novel "Angels and Demons," which was also made into a film, wrote about a secret society trying to destroy the Vatican by stealing antimatter from the European Organization for Nuclear Research (CERN). Antimatter has the same mass as the matter we know but the exact opposite electric charge. When it comes into contact with matter, they annihilate each other. That is why rumors spread that antimatter research at CERN, as in the novel's premise, could potentially swallow Earth.
Scientists believe that matter and antimatter existed in equal amounts right after the birth of the universe. Then why didn't the universe annihilate, and why is matter overwhelmingly abundant now? Observations of neutrinos traveling hundreds of kilometers in the United States and Japan have found a clue. The results put weight on so-called "matter–antimatter symmetry breaking (CP violation)," the idea that there was more matter than antimatter from the start.
◇ Ghost particles of the universe tracked in the U.S. and Japan
Japan's T2K experiment and America's NOvA experiment jointly analyzed their respective research results and announced in the international journal Nature on the 23rd that they succeeded in reducing the uncertainty of neutrino mass differences to under 2%. This was interpreted as a result that could help demonstrate matter–antimatter symmetry breaking.
Neutrinos are one of the universe's fundamental particles, with almost no mass and virtually no interactions with other matter, earning them the nickname "ghost particles." They were first proposed theoretically in the 1930s by physicist Wolfgang Pauli. They are emitted when stars explode and also appear when cosmic rays collide with atmospheric particles. They are also produced in nuclear fission at nuclear power plants and in particle accelerators.
This paper analyzes combined data from 10 years of T2K collected since 2010 and six years of NOvA collected since 2014. Two research teams that had long competed joined forces to produce results.
There are three types of neutrinos: electron neutrinos, tau neutrinos, and muon neutrinos. The T2K experiment fired muon neutrinos from the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, Ibaraki, and studied so-called neutrino oscillations in which they transformed into electron neutrinos in a massive underground tank 1 kilometer deep in Kamioka, 295 kilometers away. The project involved more than 560 people from 75 institutions in 14 countries. T2K stands for "Tokai to Kamioka."
America's NOvA is a similar experiment. From the U.S. Department of Energy's Fermi National Accelerator Laboratory near Chicago, Illinois, the team sent neutrinos to a 14,000-ton liquid scintillator detector in Ash River, Minnesota, 810 kilometers away, and observed muon neutrinos converting into electron neutrinos. More than 250 people from 49 institutions in eight countries took part.
◇ Emphasis on matter–antimatter asymmetry
Yu In-tae, a physics professor at Sungkyunkwan University, said, "The results from T2K and NOvA not only provide important clues about the mass of neutrinos, one of the fundamental particles that make up the universe, but are also significant in suggesting the possibility of matter–antimatter asymmetry in neutrinos."
The two research teams asked which neutrino is the lightest. Two explanations are possible. The normal hierarchy holds that two neutrinos are similarly light and one is heavy. The inverted hierarchy holds that two are similarly heavy and one is light.
In the normal hierarchy, the probability that a muon neutrino oscillates into an electron neutrino increases, while the probability that a muon antineutrino oscillates into an electron antineutrino decreases. In the inverted mass hierarchy, the opposite is true. The oscillation asymmetry between neutrinos and antineutrinos can also be explained if neutrinos violate CP symmetry—that is, if neutrinos do not behave identically to their antimatter counterparts.
The combined results of NOvA and T2K did not support either mass ordering. However, if future results show that the neutrino mass ordering is inverted, the results announced today would provide evidence that neutrinos violate CP symmetry and could explain why the universe is dominated by matter rather than antimatter, the teams said.
Professor Yu said, "Given these results, we expect that next-generation neutrino research facilities such as Japan's Hyper-Kamiokande or America's DUNE will discover matter–antimatter asymmetry," adding, "We anticipate being able to understand why matter exists in overwhelmingly greater amounts than antimatter in the universe."
◇ Neutrinos that have repeatedly produced Nobel Prizes
Even now, 100 billion neutrinos pass through every thumbnail-sized area of Earth each second. These ghost particles are a treasure trove of Nobel Prizes. In 1956, American physicist Reines first observed neutrinos from a nuclear power plant. He received the Nobel Prize in physics in 1995. In 1988, three American scientists received the Nobel Prize for observing neutrinos produced in a particle accelerator.
Then came Japan's golden age. University of Tokyo professor Koshiba Masatoshi observed neutrinos emitted during a supernova explosion using the first-generation Kamiokande, which held 4,500 tons of water, and received the 2002 Nobel Prize in physics. His student, Professor Kajita Takaaki, observed that atmospheric neutrinos changed type while traveling long distances using the second-generation Super-Kamiokande, an upgraded version of Kamiokande, and received the 2015 Nobel Prize in physics.
Nobel Prizes for neutrinos have been driven by large-scale scientific experimental facilities. The United States, Japan, China, and Europe are aiming for another Nobel with next-generation neutrino facilities. China began operating the Jiangmen Underground Neutrino Observatory (JUNO), the world's largest next-generation neutrino detector, in Aug. this year. The United States is also pushing the $3 billion (about 4.3 trillion won) DUNE neutrino discovery project, and Japan is building the next-generation neutrino observatory Hyper-Kamiokande.
Professor Yu said, "Japan expects a third Nobel Prize in physics in the neutrino field through the third-generation Hyper-Kamiokande experiment," adding, "In Korea, physicists and astronomers are pursuing the Korea Neutrino Observatory project to surpass Hyper-Kamiokande, but they are facing many difficulties, so active government support is urgently needed."
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-09599-3