We report measurements by the T2K experiment of the parameters $\theta_{ 23}$ and $\Delta m^2_{ 32}$ 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 taken from $1.49 \times 10^{ 21}$ protons on target (POT) in neutrino mode and $1.64 \times 10^{ 21}$ POT in antineutrino mode are used. The best-fit values obtained by T2K were $\sin^2\left(\theta_{ 23}\right)=0.51^{ +0.06}_{ -0.07} \left(0.43^{ +0.21}_{ -0.05}\right)$ and $\Delta m^2_{ 32}=2.47^{ +0.08}_{ -0.09} \left(2.50^{ +0.18}_{ -0.13}\right)$\evmass 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 non-physical 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.
Categories: hep-ex