Observation of muon neutrino disappearance with the MINOS detectors in the NuMI neutrino beam

This Letter reports results from the MINOSexperiment based on its initial exposure to neutrinos from the Fermilab NuMIbeam. The rates and energy spectra of charged currentν_μ interactions are compared in two detectors located along the beam axis at distances of 1 and 735 km. With 1.27×10^(20) 120 GeV protons incident on the NuMItarget, 215 events with energies below 30 GeV are observed at the Far Detector, compared to an expectation of 336±14 events. The data are consistent with ν_μ disappearance via oscillations with Δm_(32)^2|=2.74_(-0.26)^(+0.44)×10^(-3)  eV^2 and sin^2(2θ_(23))>0.87 (68% C.L.).