A Vμ disappearance analysis performed on the dataset collected during the first
physics run of the T2K experiment is presented. This dataset represents an
accumulated exposure of 3.23 x10¹⁹ protons on target (POT). A total of eight
events passed all the selection criteria at the far detector, Super-Kamiokande,
with an expectation of 22.8 ± 4.8(stat)+3.7-3.7(syst).
A two flavor neutrino oscillation fit was performed with systematic uncertainties using a Markov Chain Monte Carlo method and a Bayesian approach.
The sin² 2θ23 oscillation parameter was constrained to the physical region.
Under these conditions, we found best fit values for the oscillation parameters
of sin² 2θ23 = 0.83+016-0.18 and Δm²32 = 2.9+0.8-1.0 x 10⁻³ eV²/c⁴.