First observations of separated atmosphericνμandν¯μevents in the MINOS detector

The complete 5.4 kton MINOS far detector has been taking data since the beginning of August 2003 at a depth of 2070 meters water-equivalent in the Soudan mine, Minnesota. This paper presents the first MINOS observations of ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ and ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ charged-current atmospheric neutrino interactions based on an exposure of 418 days. The ratio of upward- to downward-going events in the data is compared to the Monte Carlo expectation in the absence of neutrino oscillations, giving ${R}_{\mathrm{up}/\mathrm{down}}^{\mathrm{data}}/{R}_{\mathrm{up}/\mathrm{down}}^{\mathrm{MC}}={0.62}_{\ensuremath{-}0.14}^{+0.19}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{sys}.)$. An extended maximum likelihood analysis of the observed $L/E$ distributions excludes the null hypothesis of no neutrino oscillations at the 98% confidence level. Using the curvature of the observed muons in the 1.3 T MINOS magnetic field ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ and ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ interactions are separated. The ratio of ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$ to ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ events in the data is compared to the Monte Carlo expectation assuming neutrinos and antineutrinos oscillate in the same manner, giving ${R}_{{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}/{\ensuremath{\nu}}_{\ensuremath{\mu}}}^{\mathrm{data}}/{R}_{{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}/{\ensuremath{\nu}}_{\ensuremath{\mu}}}^{\mathrm{MC}}={0.96}_{\ensuremath{-}0.27}^{+0.38}(\mathrm{stat}.)\ifmmode\pm\else\textpm\fi{}0.15(\mathrm{sys}.)$, where the errors are the statistical and systematic uncertainties. Although the statistics are limited, this is the first direct observation of atmospheric neutrino interactions separately for ${\ensuremath{\nu}}_{\ensuremath{\mu}}$ and ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}$.