The T2K experiment presents new measurements of neutrino oscillation parameters using $$19.7(16.3)\times 10^{20}$$ 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 $$4.7\times 10^{20}$$ 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 $$\sin ^2\theta _{13}$$ sin 2 θ 13 and the impact of priors on the $$\delta _{\textrm{CP}}$$ δ CP measurement. Both analyses prefer the normal mass ordering and upper octant of $$\sin ^2\theta _{23}$$ sin 2 θ 23 with a nearly maximally CP-violating phase. Assuming the normal ordering and using the constraint on $$\sin ^2\theta _{13}$$ sin 2 θ 13 from reactors, $$\sin ^2\theta _{23}=0.561^{+0.021}_{-0.032}$$ sin 2 θ 23 = 0 . 561 - 0.032 + 0.021 using Feldman–Cousins corrected intervals, and $$\varDelta {}m^2_{32}=2.494_{-0.058}^{+0.041}\times 10^{-3}~\text {eV}^2$$ Δ m 32 2 = 2 . 494 - 0.058 + 0.041 × 10 - 3 eV 2 using constant $$\varDelta \chi ^{2}$$ Δ χ 2 intervals. The CP-violating phase is constrained to $$\delta _{\textrm{CP}}=-1.97_{-0.70}^{+0.97}$$ δ CP = - 1 . 97 - 0.70 + 0.97 using Feldman–Cousins corrected intervals, and $$\delta _{\textrm{CP}}=0,\pi $$ δ CP = 0 , π is excluded at more than 90% confidence level. A Jarlskog invariant of zero is excluded at more than $$2\sigma $$ 2 σ credible level using a flat prior in $$\delta _{\textrm{CP}},$$ δ CP , and just below $$2\sigma $$ 2 σ using a flat prior in $$\sin \delta _{\textrm{CP}}.$$ sin δ CP . When the external constraint on $$\sin ^2\theta _{13}$$ sin 2 θ 13 is removed, $$\sin ^2\theta _{13}=28.0^{+2.8}_{-6.5}\times 10^{-3},$$ sin 2 θ 13 = 28 . 0 - 6.5 + 2.8 × 10 - 3 , in agreement with measurements from reactor experiments. These results are consistent with previous T2K analyses.

Tele: 925-423-7359 E.Mail: cullen1@llnl.gov Website: http://www.llnl.gov.cullen1 Tele: 925-423-7359 E.Mail: cullen1@llnl.gov Website: http://www.llnl.gov.cullen1April 20, 2004 Overview I would like to determine how accurately a variety of neutron transport code packages (code and cross section libraries) can calculate simple integral parameters, such as Keff, for systems that are sensitive to thermal neutron scattering. Since we will only consider theoretical systems, we cannot really determine absolute accuracy compared to any real system. Therefore rather than accuracy, it would be more precise to say that I would like to determine the spread in answers that we obtain from a variety of code packages. This spread should serve as an excellent indicator of how accurately we can really model and calculate such systems today. Hopefully, eventually this will lead to improvements in both our codes and the thermal scattering models that they use in the future.In order to accomplish this I propose a number of extremely simple systems that involve thermal neutron scattering that can be easily modeled and calculated by a variety of neutron transport codes. These are theoretical systems designed to emphasize the effects of thermal scattering, since that is what we are interested in studying. I have attempted to I thank Ernest Plechaty for contributing interesting and informative discussions of thermal scattering. I thank Enrico Sartori for spreading the word about this comparison, and contacting a number of people who eventually contributed to this comparison. Ground rulesI want to test each code package completely, including the important, and yet often overlooked influence of code users on code results. In order to do this I ask each participant to assume they are the local expert on a code. Someone comes to your office and asks you what your best estimate is of K-eff for a system, using thermal scattering law data or free atom scattering. They are not experts on neutron transport or your code, so they only define the geometry and materials. You, as the local code expert, must then make all decisions as far as what nuclear data to use and what input parameters to define for your code, and supply the requested K-eff. If you routinely use more than one nuclear data library, or would like to show results using a variety of input options, feel free to send more than one set of results using each of your data libraries or input options; in this case please clearly state what data and input options were used for each set of results, so that we can distinguish between your sets of results. My Simplest Possible Infinite Repeating Lattice of Uranium/Water CellsTo simulate a water-moderated, uranium fueled, thermal reactor, we can use a simple cylindrical uranium pin, centered in and surrounded by a square cell filled with water. To simulate an infinite array of cells we make the four sides of the square totally reflecting, i.e., no leakage. The third dimension of the cell, along the axis of the cylinder, can be either infinite in...

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