Measurements of neutrino oscillation parameters from the T2K experiment using 3.6 × 10²¹ protons on target

The T2K experiment presents new measurements of neutrino oscillation parameters using 19.7(16.3)×10²⁰ protons on target (POT) in (anti-)neutrino mode at the far detector (FD). Compared to the previous analysis, an additional 4.7×10²⁰ POT neutrino data was collected at the FD. Significant improvements were made to the analysis methodology, with the near-detector analysis introducing new selections and using more than double the data. Additionally, this is the first T2K oscillation analysis to use NA61/SHINE data on a replica of the T2K target to tune the neutrino flux model, and the neutrino interaction model was improved to include new nuclear effects and calculations. Frequentist and Bayesian analyses are presented, including results on sin² θ₁₃ and the impact of priors on the δ_CP measurement. Both analyses prefer the normal mass ordering and upper octant of sin² θ₂₃ with a nearly maximally CP-violating phase. Assuming the normal ordering and using the constraint on sin² θ₁₃ from reactors, sin² θ₂₃ = 0.561⁺⁰.⁰²¹₋₀.₀₃₂ using Feldman–Cousins corrected intervals, and Δm²₃₂ = 2.494⁺⁰.⁰⁴¹₋₀.₀₅₈×10−3 eV² using constant Δχ² intervals. The CP-violating phase is constrained to δCP=−1.97⁺⁰.⁹⁷₋₀.₇₀ using Feldman–Cousins corrected intervals, and δ_CP = 0,π is excluded at more than 90% confidence level. A Jarlskog invariant of zero is excluded at more than 2σ credible level using a flat prior in δCP, and just below 2σ using a flat prior in sin δ_CP. When the external constraint on sin² θ₁₃ is removed, sin² θ₁₃ = 28.0+2.8−6.5×10−3, in agreement with measurements from reactor experiments. These results are consistent with previous T2K analyses.ISSN:1434-6044ISSN:1434-605