Neutrinos

Thorsten Lux

For more than a decade, IFAE has been contributing to several key experiments in the field of neutrino physics. Currently, it participates in the long-baseline neutrino oscillation experiment T2K and the next-generation experiment Hyper-Kamiokande which will start data-taking in 2027.

2025 Activities

The T2K and NOvA collaborations have completed and published the first joint analysis of long-baseline neutrino oscillation data (Nature 646, 818–824 (2025)), combining results from the two leading accelerator-based neutrino experiments. T2K, based in Japan, and NOvA, based in the United States, use intense neutrino and antineutrino beams sent over hundreds of kilometres to study flavour oscillations and probe fundamental neutrino properties.
The joint analysis combines data from both experiments within a common statistical framework, exploiting their complementary baselines, beam energies, and detector technologies. This approach significantly improves constraints on key oscillation parameters, including the atmospheric mass-squared splitting Δm²₃₂, the mixing angle θ₂₃, and the CP-violating phase δ_CP.

The combined results provide the most precise measurement to date of Δm²₃₂ from accelerator experiments and substantially reduce degeneracies between oscillation parameters. While no definitive preference for the neutrino mass ordering is observed, the analysis shows that, assuming inverted ordering, CP-conserving values of δ_CP are excluded at the 3σ level, providing evidence for CP violation in the lepton sector under this hypothesis.
This work represents an important milestone in neutrino physics, demonstrating the power of cross-experiment collaboration and setting the stage for future high-precision measurements by next-generation experiments such as Hyper-Kamiokande and DUNE.

First differential measurement of single π⁺ production in neutrino neutral-current interactions

The IFAE Neutrino group contributed to a major measurement by the T2K Collaboration reported in Physical Review Letters (Phys. Rev. Lett. 132, L452). This work presents the first double-differential measurement of the single π⁺ production cross section in neutrino neutral-current (NC) interactions on hydrocarbon using the ND280 near detector of the T2K long-baseline neutrino oscillation experiment. The study crucially improves our experimental understanding of NC pion production, a process that is a significant background for oscillation and CP-violation measurements in accelerator neutrino experiments.

The measurement uses data recorded with an off-axis neutrino beam and focuses on kinematic distributions of outgoing π⁺ mesons (momentum and angle), reporting results in a double-differential cross section as a function of pion momentum and angle. These distributions provide detailed benchmarks for neutrino interaction models, helping to constrain systematic uncertainties that currently limit precision oscillation physics. This analysis was performed within the T2K ND280 collaboration framework, employing sophisticated event reconstruction and background subtraction techniques to isolate neutral-current single π⁺ events. The results are compared with predictions from leading neutrino interaction generators, highlighting areas where models differ from observed data — information that feeds directly into improving neutrino interaction simulations used by T2K and future experiments. Importantly, this measurement formed the core topic of the PhD thesis of César Jesús-Valls, a member of the IFAE neutrino group, demonstrating significant experimental and analytical leadership in handling ND280 data and interpreting the neutrino interaction cross sections relevant for precision oscillation studies.