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 ${\overline{\nu }}_{\mu }$ production, accumulating an exposure of $1.71\times {10}^{20}$ protons on target. In the Far Detector, 97 charged current ${\overline{\nu }}_{\mu }$ events are observed. The no-oscillation hypothesis predicts 156 events and is excluded at $6.3\sigma$. The best fit to oscillation yields $|\Delta {\overline{m}}^2|=[{3.36}_{-0.40}^{+0.46}(\mathrm{stat})\pm 0.06(\mathrm{syst})]\times {10}^{-3}{\mathrm{eV}}^2$, ${sin}^2(2\overline{\theta })={0.86}_{-0.12}^{+0.11}(\mathrm{stat})\pm 0.01(\mathrm{syst})$. The MINOS ${\nu }_{\mu }$ and ${\overline{\nu }}_{\mu }$ measurements are consistent at the 2.0% confidence level, assuming identical underlying oscillation parameters.