Measurements of neutrino oscillation parameters from the T2K experiment using 3.6 x 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 δCP measurement. Both analyses prefer the normal mass ordering and upper octant of sin2 θ23 with a nearly maximally CPviolating 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 Δm2 32 = 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 δCP = 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 δCP. 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