Prof Morgan Wascko

Abstract:

Scattering of high energy particles from nucleons probes their structure, as was done in the experiments that established the non-zero size of the proton using electron beams¹. The use of charged leptons as scattering probes enables measuring the distribution of electric charges, which is encoded in the vector form factors of the nucleon². Scattering weakly interacting neutrinos gives the opportunity to measure both vector and axial vector form factors of the nucleon, providing an additional, complementary probe of their structure. The nucleon transition axial form factor, F_A, can be measured from neutrino scattering from free nucleons, ν_μn → μ^-p and [Formula: see text], as a function of the negative four-momentum transfer squared (Q²). Up to now, F_A(Q²) has been extracted from the bound nucleons in neutrino-deuterium scattering^3-9, which requires uncertain nuclear corrections¹⁰. Here we report the first high-statistics measurement, to our knowledge, of the [Formula: see text] cross-section from the hydrogen atom, using the plastic scintillator target of the MINERvA¹¹ experiment, extracting F_A from free proton targets and measuring the nucleon axial charge radius, r_A, to be 0.73 ± 0.17 fm. The antineutrino-hydrogen scattering presented here can access the axial form factor without the need for nuclear theory corrections, and enables direct comparisons with the increasingly precise lattice quantum chromodynamics computations^12-15. Finally, the tools developed for this analysis and the result presented are substantial advancements in our capabilities to understand the nucleon structure in the weak sector, and also help the current and future neutrino oscillation experiments^16-20 to better constrain neutrino interaction models.

Abstract:

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 sin2θ13 and the impact of priors on the δCP measurement. Both analyses prefer the normal mass ordering and upper octant of sin2θ23 with a nearly maximally CP-violating phase. Assuming the normal ordering and using the constraint on sin2θ13 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 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 sinδCP. When the external constraint on sin2θ13 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.