First observations of separated atmospheric<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>ν</mml:mi><mml:mi>μ</mml:mi></mml:msub></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mover accent="true"><mml:mi>ν</mml:mi><mml:mo>¯</mml:mo></mml:mover><mml:mi>μ</mml:mi></mml:msub></mml:math>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 and 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 data up=down =R MC up=down 0:62 0:19 0:14 stat: 0:02sys:. 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 and interactions are separated. The ratio of to events in the data is compared to the Monte Carlo expectation assuming neutrinos and antineutrinos oscillate in the same manner, giving R data = =R MC = 0:96 0:38 0:27 stat: 0:15sys:, 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 and .