Measurement of Neutrino Oscillation Parameters from Muon Neutrino Disappearance with an Off-Axis Beam

The T2K Collaboration reports a precision measurement of muon neutrino disappearance with an off-axis neutrino beam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrino flux and cross section parameters. The Super-Kamiokande far detector, which is 295 km downstream of the neutrino production target, collected data corresponding to $3.01\times {10}^{20}$ protons on target. In the absence of neutrino oscillations, $205\pm 17$ (syst) events are expected to be detected while only 58 muon neutrino event candidates are observed. A fit to the neutrino rate and energy spectrum, assuming three neutrino flavors and normal mass hierarchy yields a best-fit mixing angle ${sin}^2({\theta }_{23})=0.514\pm 0.082$ and mass splitting $|\Delta m_{32}^2|={2.44}_{-0.15}^{+0.17}\times {10}^{-3}{\mathrm{eV}}^2/c^4$. Our result corresponds to the maximal oscillation disappearance probability.

Figure 1

The ND280 momentum data distributions of (a) the CCQE and (b) CCnQE selections. The predicted total, CCQE, CCnQE, and background event distributions from the ND280 fit are overlaid on both figures.

Figure 2

The 58 event 1-ring $\mu$-like SK reconstructed energy spectrum. Top: The two predicted curves are the no-oscillation hypothesis and the best fit from the primary oscillation analysis. The energy scale is given on the top (0–6 GeV). Bottom: The ratio of the observed spectrum over the no-oscillation hypothesis and ratio of the best-fit curve over the no-oscillation hypothesis in two energy ranges: lower left (0–6 GeV) and lower right (0.3–1.0 GeV). The fit uses finer binning than is shown here.

Figure 3

The 68% and 90% C.L. contour regions for ${sin}^2({\theta }_{23})$ and $|\Delta m_{32}^2|$ are shown for the primary analysis. The one-dimensional profile likelihoods for each oscillation parameter are also shown separately.

Figure 4

The 90% C.L. contour region for ${sin}^2(2{\theta }_{23})$ and $|\Delta m_{32}^2|$ for the primary T2K analysis is calculated by the profiling over the octant. The T2K 2011 [3], SK [27], and MINOS [6] 90% C.L. contours with different flavor assumptions are shown for comparison.