It is now widely accepted that the Standard Model assumption of massless neutrinos is wrong, due primarily to the observation of solar and atmospheric neutrino flavor oscillations by a small number of convincing experiments. The MINOS Far Detector, capable of observing both the outgoing lepton and associated showering products of a neutrino interaction, provides an excellent opportunity to independently search for an oscillation signature in atmospheric neutrinos. To this end, a MINOS data set from an 883 live day, 13.1 kt-yr exposure collected between July, 2003 and April, 2007 has been analyzed. 105 candidate charged current muon neutrino interactions were observed, with 120.5 (+-) 1.3 (statistical error only) expected in the absence of oscillation. A maximum likelihood analysis of the observed log(L/E) spectrum shows that the null oscillation hypothesis is excluded at over 96% confidence and that the best fit oscillation parameters are sin(sup 2) 2(theta)(sub 23) = 0.95(sub -0.32) and (Delta)m(sub 23)(sup 20)= 0.93(sub -0.44)(sup +3.94) x 10(sup -3) eV(sup 2). This measurement of oscillation parameters is consistent with the best fit values from the Super-Kamiokande experiment at 68% confidence.
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