In a landmark collaboration, researchers from the T2K experiment in Japan and NOvA in the United States have merged their data for the most precise study yet of neutrino oscillations. This joint analysis, published in Nature, advances understanding of why matter prevailed over antimatter in the early universe. The effort highlights the power of international teamwork in probing cosmic mysteries.
Physicists have long puzzled over the universe's asymmetry: the early cosmos should have had equal amounts of matter and antimatter, which would have annihilated each other, leaving nothing behind. Yet matter endured, forming stars, planets, and life. Neutrinos, elusive particles that rarely interact with matter, may hold the key through their oscillation—changing flavors as they travel—and potential violation of charge-parity (CP) symmetry.
For the first time, the T2K and NOvA collaborations combined eight years of NOvA data with a decade of T2K results, starting their joint analysis in 2019. T2K, involving over 560 members from 75 institutions across 15 nations, sends neutrino beams from Japan over 295 kilometers to detectors. NOvA, with more than 250 scientists from 49 institutions in eight countries, beams particles from Fermilab in the US to a detector in Minnesota, 810 kilometers away. These long-baseline setups complement each other, yielding unprecedented precision in measuring oscillations.
"This was a big victory for our field," said Kendall Mahn, a Michigan State University professor and T2K co-spokesperson. "This shows that we can do these tests, we can look into neutrinos in more detail and we can succeed in working together."
The study focuses on neutrino mass ordering: whether it follows a normal hierarchy (two light, one heavy) or inverted (two heavy, one light). A CP violation could explain matter's dominance, but results remain inconclusive, favoring neither ordering decisively. "Neutrinos are not well understood," noted MSU postdoctoral associate Joseph Walsh. "Their very small masses mean they don't interact very often. Hundreds of trillions of neutrinos from the sun pass through your body every second, but they will almost all pass straight through."
"By making a joint analysis you can get a more precise measurement than each experiment can produce alone," added NOvA collaborator Liudmila Kolupaeva.
While not resolving the mystery, the findings, detailed in Nature (DOI: 10.1038/s41586-025-09599-3), bolster future probes into neutrinos' role in the universe's evolution. As T2K collaborator Tomáš Nosek stated, "These results are an outcome of a cooperation and mutual understanding of two unique collaborations."