Neutrino oscillations measured by the T2K and NOvA collaborations

Ultra-light elementary particles, neutrinos behave in a curious way. When they move, they change “costume” (scientists refer to this as “flavor”) and transform from one type of neutrino to another. For example, muon neutrinos can become tau neutrinos or even electron neutrinos.

This phenomenon, known as neutrino oscillation, was first observed in the 1990s. Today, large international collaborations, each involving hundreds of researchers, are still attempting to observe these oscillations with ever greater precision. The aim is to learn more about the masses of neutrinos, but also to resolve one of the great questions about our Universe: why is there more matter than antimatter?

Antimatter particles are, in a way, twins of matter particles, but with some “reversed” characteristics, such as their electric charge. For example, an “antielectron” (or positron) is similar to an electron, but with a positive charge.

A possible asymmetry between matter and antimatter

Our universe today contains very little antimatter. However, based on what physicists know about the Big Bang, matter and antimatter should have been created in equal proportions... Could there be a slight asymmetry between the behavior of matter and that of antimatter that could explain the observed imbalance?

“This is why we compare the oscillations of neutrinos with those of antineutrinos in search of what we call a violation of charge-parity symmetry,” explains Margherita Buizza Avanzini, a researcher at the Leprince-Ringuet Laboratory (LLR*) involved in the T2K collaboration (Japan).

An acronym for Tokai to Kamioka, the experiment involves producing a beam of muon neutrinos (or muon antineutrinos for comparison) using an accelerator located in the city of Tokai, then sending these particles nearly 300 km away to the Kamioka region, where the Super-Kamiokande detector is located. The latter observes what has become of the particles, in particular their change through oscillation. This is particularly difficult because neutrinos interact very little, making them very difficult to measure! The T2K collaboration has been in existence for almost 20 years, and the total number of particles detected on arrival amounts to only several hundred.

Together with the NOvA collaboration, a similar experiment taking place in the United States, the members of T2K have just published their joint analysis in Nature. “The initial goal was to set up a process for pooling data from the two competing collaborations,” explains Margherita Buizza Avanzini. 

The joint analysis combined ten years of T2K data collected since 2010 and six years of NOvA data since 2014. The results are not yet significant enough to be certain of a violation of charge-parity symmetry, although they point in that direction with increased precision. The announcement could come within a few years. In the meantime, both experiments continue to collect data. The Hyper-Kamiokande detector, an upgraded version of Super-Kamiokande, is expected to come online in 2028.

Article reference:

The NOvA Collaboration., The T2K Collaboration. Joint neutrino oscillation analysis from the T2K and NOvA experiments. Nature 646, 818–824 (2025). https://doi.org/10.1038/s41586-025-09599-3

*LLR: a joint research unit CNRS, École Polytechnique, Institut Polytechnique de Paris, 91120 Palaiseau, France