First direct observation of muon antineutrino disappearance.
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This Letter reports the first direct observation of muon antineutrino disappearance. The MINOS experiment has taken data with an accelerator beam optimized for () production, accumulating an exposure of 1.71 10 protons on target. In the Far Detector, 97 charged current () events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at 6.3. The best fit to oscillation yields |m| = [3.36(-0.40)(+0.46)(stat) 0.06(syst)] 10 eV, sin(2) = 0.86(-0.12)(+0.11)(stat) 0.01(syst). The MINOS () and () measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.