Measurement of Neutrino Oscillation Parameters from Muon Neutrino Disappearance with an Off-Axis Beam

摘要

The T2K Collaboration reports a precision measurement of muon neutrino disappearance with an off-axis neutrino beam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrino flux and cross section parameters. The Super-Kamiokande far detector, which is 295 km downstream of the neutrino production target, collected data corresponding to 3.01 X 1020 protons on target. In the absence of neutrino oscillations, 205 ±17 (syst) events are expected to be detected while only 58 muon neutrino event candidates are observed. A fit to the neutrino rate and energy spectrum, assuming three neutrino flavors and normal mass hierarchy yields a best-fit mixing angle sin~2(θ_(23))＝ 0.514 ± 0.082 and mass splitting |Δm_(32)~2| ＝ 2.44_(-0.15)~(+0.17) × 10~(-3) eV~2/c~4. Our result corresponds to the maximal oscillation disappearance probability.