First observations of separated atmospheric νμ and (ν)over-barμ events 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 nu(mu) and (nu) over bar (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-up/down(data)/R-up/down(MC)=0.62(-0.14)(+0.19)(stat.)+/- 0.02(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 nu(mu) and (nu) over bar (mu) interactions are separated. The ratio of (nu) over bar (mu) to nu(mu) events in the data is compared to the Monte Carlo expectation assuming neutrinos and antineutrinos oscillate in the same manner, giving R-(nu) over bar mu/nu(data)(mu)/R-(nu) over bar mu/nu(MC)(mu)=0.96(-0.27)(+0.38)(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 neutrino interactions separately for nu(mu) and (nu) over bar (mu).