  T2K reports its first measurements of the parameters governing the
disappearance of $\bar{\nu}_\mu$ in an off-axis beam due to flavor change
induced by neutrino oscillations. The quasimonochromatic $\bar{\nu}_\mu$ 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 $\bar{\nu}_\mu$ 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 $\sin^2 (\bar{\theta}_{23}) =
0.45$ and $|\Delta\bar{m}^2_{32}| = 2.51 \times 10^{-3}$ 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 $\nu_\mu$ disappearance parameters
measured by T2K.
