This letter reports the first direct observation of muon antineutrino disappearance. The MINOS experiment has taken data with an accelerator beam optimized for muon antineutrino production, accumulating an exposure of $1.71times 10^{20}$ protons on target. In the Far Detector, 97 charged current muon antineutrino events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at $6.3sigma$. The best fit to oscillation yields $Delta bar{m}^{2}=(3.36^{+0.46}_{-0.40}textrm{(stat.)}pm0.06textrm{(syst.)})times 10^{-3},eV^{2}$, $sin^{2}(2bar{theta})=0.86^{+0.11}_{-0.12}textrm{(stat.)}pm0.01textrm{(syst.)}$. The MINOS muon neutrino and muon antineutrino measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.
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