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