Measurement of Muon Antineutrino Oscillations with an Accelerator-Produced Off-Axis Beam

T2K reports its first measurements of the parameters governing the disappearance of ${\overline{\nu }}_{\mu }$ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic ${\overline{\nu }}_{\mu }$ beam, produced with a peak energy of 0.6 GeV at J-PARC, is observed at the far detector Super-Kamiokande, 295 km away, where the ${\overline{\nu }}_{\mu }$ survival probability is expected to be minimal. Using a data set corresponding to $4.01\times {10}^{20}$ protons on target, 34 fully contained $\mu$-like events were observed. The best-fit oscillation parameters are ${\sin }^2({\overline{\theta }}_{23})=0.45$ and $|\mathrm{\Delta }{\overline{m}}_{32}^2|=2.51\times {10}^{-3}{\mathrm{eV}}^2$ with 68% confidence intervals of $0.38–0.64$ and $2.26–2.80\times {10}^{-3}{\mathrm{eV}}^2$, respectively. These results are in agreement with existing antineutrino parameter measurements and also with the ${\nu }_{\mu }$ disappearance parameters measured by T2K.

Figure 1

The nominal unoscillated neutrino flux prediction at SK for each neutrino type in the neutrino-mode beam (left) and antineutrino-mode beam (right). The shaded boxes indicate the total systematic uncertainty on each energy bin.

Figure 2

The momentum (left) and angular (right) distributions of the muon candidates at ND280 from the ${\nu }_{\mu }$ CC $0\pi$ (top), the ${\overline{\nu }}_{\mu }$ CC 1 track (center) and the ${\nu }_{\mu }$ CC 1 track (bottom) samples. The data are superimposed on the post-fit MC prediction, separated by interaction mode.

Figure 3

Top: The reconstructed energy distribution of the 34 far-detector ${\overline{\nu }}_{\mu }$ candidates and the best-fit prediction, separated by interaction mode. This is compared to the predicted spectrum assuming the antineutrino oscillation parameters are identical to the neutrino parameters measured by T2K [13]. Bottom: The observed data and ${\overline{\nu }}_{\mu }$-mode best-fit prediction as a ratio to the unoscillated prediction.

Figure 4

The 68% and 90% confidence regions for the ${\sin }^2({\overline{\theta }}_{23})-|\mathrm{\Delta }{\overline{m}}_{32}^2|$ plane assuming normal hierarchy, alongside the T2K $\nu$ [13], SK Collaboration $\overline{\nu }$ [5], and MINOS $\overline{\nu }$ [4] 90% confidence regions. One-dimensional $\mathrm{\Delta }{\chi }^2$ profiles for the two parameters are shown at the top and right, overlaid with lines representing the 1D $\mathrm{\Delta }{\chi }^2$ values for the 68% and 90% confidence intervals.