| Abstract: | T2K reports its first measurements of the parameters governing the
disappearance of $overline{
u}_mu$ in an off-axis beam due to flavour change
induced by neutrino oscillations. The quasi-monochromatic $overline{
u}_mu$
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 $overline{
u}_mu$ survival
probability is expected to be minimal. Using a dataset corresponding to $4.01
imes 10^{20}$ protons on target, $34$ fully contained $mu$-like events were
observed. The best-fit oscillation parameters are: $sin^2
(overline{ heta}_{23}) = 0.45$ and $|Deltaoverline{m}^2_{32}| = 2.51 imes
10^{-3}$ eV$^2$ with 68% confidence intervals of 0.38 - 0.64 and 2.26 - 2.80
($ imes 10^{-3}$ eV$^2$) respectively. These world-leading results are in
agreement with existing anti-neutrino parameter measurements and also with the
$
u_mu$ disappearance parameters measured by T2K. |
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