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.