We report measurements by the T2K experiment of the parameters theta23 and Delta_m32^2, which govern the disappearance of muon neutrinos and antineutrinos in the three-flavor PMNS neutrino oscillation model at T2K's neutrino energy and propagation distance. Utilizing the ability of the experiment to run with either a mainly neutrino or a mainly antineutrino beam, muon-like events from each beam mode are used to measure these parameters separately for neutrino and antineutrino oscillations. Data

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... taken from 1.49×10^21 protons on target (POT) in neutrino mode and 1.64×10^21 POT in antineutrino mode are used. The best-fit values obtained by T2K were sin^2(θ23)=0.51-0.07+0.06(0.43-0.05+0.21) and Delta_m32^2=2.47-0.09+0.08(2.50-0.13+0.18)×10^-3 e^V2/c^4 for neutrinos (antineutrinos). No significant differences between the values of the parameters describing the disappearance of muon neutrinos and antineutrinos were observed. An analysis using an effective two-flavor neutrino oscillation model where the sine of the mixing angle is allowed to take nonphysical values larger than 1 is also performed to check the consistency of our data with the three-flavor model. Our data were found to be consistent with a physical value for the mixing angle.