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 (bar ν)μ production, accumulating an exposure of 1.71 x 1020 protons on target. In the Far Detector, 97 charged current (bar ν)μ events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at 6.3σ. The best fit to oscillation yields |Δ(bar m)2| = (3.36-0.40+0.46(stat.) ± 0.06(syst.)) x 10-3 eV2, sin2(2(bar θ)) = 0.86-0.12+0.11(stat.) ± 0.01(syst.). The MINOS νμ and (bar ν)μ measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.