The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle $theta_{13}$ have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation
experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal $sin^22theta_{23}$, the octant of $theta_{23}$, and the mass hierarchy, in addition to the measurements of $delta_{CP}$, $sin^2theta_{23}$, and $Delta m^2_{32}$, for various combinations of $ u$-mode and (bar{ u})-mode data-taking. With an exposure of $7.8times10^{21}$~protons-on-target, T2K can achieve 1-$sigma$ resolution of 0.050(0.054) on $sin^2theta_{23}$ and $0.040(0.045)times10^{-3}~rm{eV}^2$ on $Delta m^2_{32}$ for 100%(50%) neutrino beam mode running assuming $sin^2theta_{23}=0.5$ and $Delta m^2_{32} = 2.4times10^{-3}$ eV$^2$. T2K will have sensitivity to the CP-violating phase $delta_{rm{CP}}$ at 90% C.L. or better over a significant range. For example, if $sin^22theta_{23}$ is maximal (i.e $theta_{23}$=$45^circ$) the range is $-115^circ<delta_{rm{CP}}<-60^circ$ for normal hierarchy and $+50^circ<delta_{rm{CP}}<+130^circ$ for inverted hierarchy. When T2K data is combined with data from the NO$ u$A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero $delta_{CP}$ is substantially increased compared to if each experiment is analyzed alone.
The Tokai-to-Kamioka (T2K) experiment studies neutrino oscillations using an off-axis muon neutrino beam with a peak energy of about 0.6 GeV that originates at the J-PARC accelerator facility. Interactions of the neutrinos are observed at near detect
ors placed at 280 m from the production target and at the far detector -- Super-Kamiokande (SK) -- located 295 km away. The flux prediction is an essential part of the successful prediction of neutrino interaction rates at the T2K detectors and is an important input to T2K neutrino oscillation and cross section measurements. A FLUKA and GEANT3 based simulation models the physical processes involved in the neutrino production, from the interaction of primary beam protons in the T2K target, to the decay of hadrons and muons that produce neutrinos. The simulation uses proton beam monitor measurements as inputs. The modeling of hadronic interactions is re-weighted using thin target hadron production data, including recent charged pion and kaon measurements from the NA61/SHINE experiment. For the first T2K analyses the uncertainties on the flux prediction are evaluated to be below 15% near the flux peak. The uncertainty on the ratio of the flux predictions at the far and near detectors is less than 2% near the flux peak.
