Measurements of neutrino oscillation parameters from the T2K experiment using 3.6 × 10 ²¹ protons on target

The T2K experiment presents new measurements of neutrino oscillation parameters using 19.7 (16.3) × 10 ²⁰ protons on target (POT) in (anti-)neutrino mode at the far detector (FD). Compared to the previous analysis, an additional 4.7 × 10 ²⁰ 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 sin ²θ₁₃ and the impact of priors on the δ_CP measurement. Both analyses prefer the normal mass ordering and upper octant of sin ²θ₂₃ with a nearly maximally CP-violating phase. Assuming the normal ordering and using the constraint on sin ²θ₁₃ from reactors, sin2θ23=0.561-0.032+0.021 using Feldman–Cousins corrected intervals, and Δm322=2.494-0.058+0.041×10-3eV2 using constant Δχ² intervals. The CP-violating phase is constrained to δCP=-1.97-0.70+0.97 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 2 σ credible level using a flat prior in δ_CP, and just below 2 σ using a flat prior in sin δ_CP. When the external constraint on sin ²θ₁₃ is removed, sin2θ13=28.0-6.5+2.8×10-3, in agreement with measurements from reactor experiments. These results are consistent with previous T2K analyses.