First direct observation of muon antineutrino disappearance

This Letter reports the first direct observation of muon antineutrino disappearance. The MINOS experiment has taken data with an accelerator beam optimized for ?¯ µ production, accumulating an exposure of 1.71×1020 protons on target. In the Far Detector, 97 charged current ?¯ µ events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at 6.3s. The best fit to oscillation yields |?m¯ 2|=[3.36-0.40+0.46(stat)±0.06(syst)]×10-3??eV2, sin?2(2?¯ )=0.86-0.12+0.11(stat)±0.01(syst). The MINOS ?µ and ?¯ µ measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.