In both NOvA (NuMI Off-axis νe Appearance experiment) and T2K experiments, neutrinos are fired from particle accelerators and detected after traveling long distances underground. The challenge is immense: out of trillions upon trillions of particles, only a handful leave detectable traces. Sophisticated detectors and software then reconstruct these rare events, providing clues about how neutrinos change ‘flavor’ as they travel move.
The world’s first neutrino observation in a hydrogen bubble chamber. It was found November 13, 1970, on this photograph from the Zero Gradient Synchrotron’s 12-foot bubble chamber. The invisible neutrino strikes a proton where three particle tracks originate (lower right). The neutrino turns into a mu-meson, the long center track (extending up and left). The short track is the proton. The third track (extending down and left) is a pi-meson created by the collision. Image credit: Argonne National Laboratory.
Neutrinos are among the most abundant particles in the Universe.
They have no electric charge and nearly no mass, making them extraordinarily difficult to detect. But that same elusiveness makes them scientifically priceless.
Understanding neutrinos could help explain one of the greatest puzzles in cosmology: why the Universe is made of matter.
Theoretically, the Big Bang should have produced equal parts matter and antimatter, which would have annihilated each other completely; when a particle meets its mirror opposite, both disappear in a burst of energy.
But when the Big Bang occurred something tipped the balance, creating a greater abundance of matter, which led to the formation of stars, galaxies, and life today.
Physicists suspect that neutrinos may hold the answer.
Neutrinos come in three types, or ‘flavors,’ electron, muon, and tau, essentially three versions of the same tiny particle.
They possess the unusual ability to oscillate and transform from one flavor to another as they travel through space, and the way these oscillations occur, and whether they differ between neutrinos and their antimatter counterparts, could reveal why matter won out over antimatter in the early Universe.
“Understanding these different identities can help scientists learn more about neutrino masses and answer key questions about the evolution of the Universe, including why matter came to dominate over antimatter in the early Universe,” said Dr. Zoya Vallari, a physicist at the Ohio State University.
“The reason neutrinos are really, really fun is because they change their flavors.”
“Imagine getting chocolate ice cream, walking down the street, and suddenly it turns into mint, and every time it moves, it changes again.”
In an effort to better understand this shape-shifting behavior, NOvA and T2K experiments combined forces to shoot beams of neutrino particles over hundreds of km.
NOvA sends a beam of neutrinos through the Earth 810 km from its source at the Fermi National Accelerator Laboratory near Chicago to a 14,000-ton detector in Ash River, Minnesota.
Japan’s T2K shoots a beam of neutrinos 295 km from the J-PARC accelerator in Tokai to the giant Super-Kamiokande detector under Mount Ikenoyama.
“While our goals were the same, differences in our experiment design adds more information when we pool our data together, in that the sum is more than its parts,” Dr. Vallari said.
While this study builds on previous work that found tiny, but still very consequential, differences in neutrino mass for each type, the researchers sought deeper hints that neutrinos operate outside the standard laws of physics.
One such question is whether neutrinos and their antimatter counterparts behave differently, a phenomenon called Charge-Parity violation.
“Our results show that we need more data to be able to significantly answer these fundamental questions,” Dr. Vallari said.
“That’s why building the next generation of experiments is important.”
According to the study, combining the results of both experiments allowed the scientists to get a handle on these pressing physics questions from different angles, as two experiments with different baselines and energies have a better chance of answering them than a single experiment alone.
“This work is extraordinarily complex, and each collaboration involves hundreds of people,” said Ohio State University’s Professor John Beacom.
“Collaborations like these are usually competing, so that they are co-operating here shows how high the stakes are.”
The new findings were published in the journal Nature.
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NOvA Collaboration & T2K Collaboration. 2025. Joint neutrino oscillation analysis from the T2K and NOvA experiments. Nature 646, 818-824; doi: 10.1038/s41586-025-09599-3
