T2K reports its first measurements of the parameters governing the disappearance of $bar{ u}_mu$ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic $bar{ u}_mu$ 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 $bar{ u}_mu$ 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 $sin^2 (bar{theta}_{23}) = 0.45$ and $|Deltabar{m}^2_{32}| = 2.51 times 10^{-3}$ eV$^2$ with 68% confidence intervals of 0.38 - 0.64 and 2.26 - 2.80 $times 10^{-3}$ eV$^2$ respectively. These results are in agreement with existing antineutrino parameter measurements and also with the $ u_mu$ disappearance parameters measured by T2K.
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