Precise Measurement of the Neutrino Mixing Parameter ${\theta }_{23}$ from Muon Neutrino Disappearance in an Off-Axis Beam

New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter ${\theta }_{23}$. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to $6.57\times 10^{20}$ protons on target, T2K has fit the energy-dependent ${\nu }_{\mu }$ oscillation probability to determine oscillation parameters. The 68% confidence limit on ${\sin }^2({\theta }_{23})$ is $0.514_{-0.056}^{+0.055}$ ($0.511\pm 0.055$), assuming normal (inverted) mass hierarchy. The best-fit mass-squared splitting for normal hierarchy is $\mathrm{\Delta }{m}_{32}^2=(2.51\pm 0.10)\times {10}^{-3}{\mathrm{eV}}^2/c^4$ (inverted hierarchy: $\mathrm{\Delta }{m}_{13}^2=(2.48\pm 0.10)\times {10}^{-3}{\mathrm{eV}}^2/c^4$). Adding a model of multinucleon interactions that affect neutrino energy reconstruction is found to produce only small biases in neutrino oscillation parameter extraction at current levels of statistical uncertainty.

Figure 1

The momentum and angular distributions for muons in ND280’s $\mathrm{CC}\text{−}0\pi$ selection. The predicted distributions before and after the ND280 fit are overlaid on both figures.

Figure 2

Reconstructed energy spectrum for single-ring $\mu$-like SK events. Top: The observed spectrum and expected spectrum with interaction modes for the T2K best fit. Bottom: The ratio of the observed spectrum (points) to the no-oscillation hypothesis, and the best oscillation fit (solid).

Figure 3

The 68% and 90% C.L. confidence regions for ${\sin }^2({\theta }_{23})$ and $\mathrm{\Delta }{m}_{32}^2$ (NH) or $\mathrm{\Delta }{m}_{13}^2$ (IH). The SK [49] and MINOS [7] 90% C.L. regions for NH are shown for comparison. T2K’s 1D profile likelihoods for each oscillation parameter separately are also shown at the top and right overlaid with light blue lines and points representing the 1D $-2\mathrm{\Delta }\ln {\mathcal{L}}_{\text{critical}}$ values for NH at 68% and 90% C.L.

Figure 4

The difference between the reconstructed energy assuming QE kinematics and the true neutrino energy. True QE events with energies below 1.5 GeV show little bias while multinucleon events based on [43] and NEUT pionless $\mathrm{\Delta }$ decay (shown scaled up by a factor of 5) are biased towards lower energies.