First direct observation of muon antineutrino disappearance

This Letter reports the first direct observation of muon antineutrino disappearance. The MINOSexperiment has taken data with an accelerator beam optimized for ν _μ production, accumulating an exposure of 1.71×10^(20) protons on target. In the Far Detector, 97 charged currentν _μ events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at 6.3σ. The best fit to oscillation yields |Δm 2|= [3.36=_(-0.40)^(+0.46)( stat)±0.06(syst)]x10^(-3)eV^2,sin^2(2θ)=0.86 _(-0.12)^(+0.11)( stat)±0.01(syst). The MINOSν _μ and ν _μ measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.