First observations of separated atmospheric nu(mu) and anti-nu(mu) events in the MINOS detector.

The complete 5.4 kton MINOSfar detector has been taking data since the beginning of August 2003 at a depth of 2070 meters water-equivalentin the Soudan mine, Minnesota. This paper presents the first MINOSobservations of muon neutrinoand muonanti- neutrino charged-currentatmospheric neutrinointeractions 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 oscillationsgiving: R_data(up/down)/R_MC(up/down) = 0.62^{+0.19}_{-0.14} (stat.) +- 0.02 (sys.). An extended maximum likelihood analysis of the observed L/E distributions excludes the null hypothesis of no neutrino oscillationsat the 98 % confidence level. Using the curvature of the observed muonsin the 1.3 T MINOSmagnetic field muon neutrinoand muonanti- neutrinointeractions are separated. The ratio of muon neutrinoto muonanti- neutrinoevents in the data is compared to the Monte Carlo expectation assuming neutrinosand anti- neutrinos oscillatein same manner giving: R_data(numubar/numu)/R_MC(numubar/numu) = 0.96^{+0.38}_{-0.27} (stat.) +- 0.15 (sys.), where the errors are the statistical and systematic uncertainties. Although the statistics are limited, this is the first direct observation of atmospheric neutrinointeractions separately for muon neutrinosand muonanti- neutrinos.