[Research Highlights] T2K and Super-Kamiokande join forces to study neutrino oscillations

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The Super-Kamiokande and T2K collaborations have produced a joint analysis of their data for the study of standard neutrino oscillations. The results were published as “Editors’ Suggestion” in Physical Review Letters.

Both experiments have individually been studying neutrino oscillations for a long time. Super- Kamiokande studies the oscillations of solar and atmospheric neutrinos, and realized in particular the first experimental observation of neutrino oscillations. T2K studies neutrinos produced using an accelerator, and realized the first observation of the appearance of a neutrino flavor through oscillations. The experiments are now looking at two of the main open questions in the field: whether neutrinos and anti-neutrinos oscillate in the same way (“CP symmetry”) and what the ordering of the neutrino mass states is (“mass ordering”). Answering these questions requires precise measurements of neutrino oscillations, and a combined analysis of T2K accelerator neutrinos and Super-Kamiokande atmospheric neutrinos is an appealing way to achieve this. Such a joint analysis has increased precision due to the combination of complementary datasets and the ability to constrain some of the uncertainties affecting one experiment using the dataset from the other one. However, building a proper joint analysis requires to consider the correlations between the systematic uncertainties of the two experiments. This was addressed by unifying the analyses of the two experiments where needed, building in particular a common neutrino interaction model for the events that have similar energies between the two experiments, and a common detector response uncertainty model. This first combined analysis of beam and atmospheric neutrinos is an important step towards the development of combined analyses for the next generation of neutrino experiments (such as Hyper-Kamiokande) which will use larger detectors to give definite answers to the questions of neutrino mass ordering and violation of CP symmetry in neutrino oscillations.

The paper presents the results of four different analyses giving consistent results. The datasets from the two experiments can properly be described by the common model built for this analysis, and preferences for the normal neutrino mass ordering and non conservation of CP symmetry were found. These preferences are stronger when using the combined dataset than when using only one of the individual datasets from the two experiments. The significance of the non-conservation of CP symmetry is evaluated using new methods, and found to be between 1.9σ and 2.0σ depending on the analysis considered.

 

Constraints on neutrino oscillation parameters obtained using different datasets: T2K only, SK only and the combination of T2K and SK datasets

 

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This research publication:

First Joint Oscillation Analysis of Super-Kamiokande Atmospheric and T2K Accelerator Neutrino Data, Super-Kamiokande Collaboration and T2K Collaboration,

Phys. Rev. Lett. 134, 011801 (2025)
arXiv:2405.12488

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