Measurement of Muon Antineutrino Oscillations with an Accelerator-Produced Off-Axis Beam
2016
T2K reports its first measurements of the parameters governing the disappearance of $\
overline{\nu}_\mu$ in an off-axis beam due to flavour change induced by
neutrino oscillations. The quasi-monochromatic $\
overline{\nu}_\mu$ 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 dataset 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 $|\Delta\
overline{m}^2_{32}| = 2.51 \times 10^{-3}$ eV$^2$ with 68% confidence intervals of 0.38 - 0.64 and 2.26 - 2.80 ($\times 10^{-3}$ eV$^2$) respectively. These world-leading results are in agreement with existing anti-neutrino parameter measurements and also with the $\nu_\mu$ disappearance parameters measured by T2K.
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