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.

Figure 1

The reconstructed energy spectra of events in the Near Detector classified as charged current interactions, separated according to the reconstructed charge of the track. The events with a negatively charged track are not used in the oscillation analysis. The shaded bands represent the systematic uncertainty on the simulation.

Figure 2

The distribution of the sign of the reconstructed charge divided by the momentum of selected muon tracks in the Far Detector. The simulated distribution is shown in the case of no oscillation and oscillation assuming the best-fit ${\nu }_{\mu }$ parameters from Ref. [3] and ${\overline{\nu }}_{\mu }$ parameters from this analysis.

Figure 3

Comparison of the measured Far Detector ${\overline{\nu }}_{\mu }$ CC energy spectrum to the expectation in three cases: in the absence of oscillation, using the oscillation parameters which best fit these ${\overline{\nu }}_{\mu }$ data (for this case, the total expected background is also indicated), and using the best-fit ${\nu }_{\mu }$ oscillation parameters measured by MINOS [3].

Figure 4

Allowed regions for the ${\overline{\nu }}_{\mu }$ oscillation parameters from a fit to the data in Fig. 3, including all sources of systematic uncertainty. Indirect limits prior to this work [19] and the MINOS allowed region for ${\nu }_{\mu }$ oscillation [3] are also shown.