The T2K experiment observes indications of ${\nu }_{\mu }\to {\nu }_e$ appearance in data accumulated with $1.43\times {10}^{20}$ protons on target. Six events pass all selection criteria at the far detector. In a three-flavor neutrino oscillation scenario with $|\Delta m_{23}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$, ${sin}^{2}2{\theta }_{23}=1$ and ${sin}^{2}2{\theta }_{13}=0$, the expected number of such events is $1.5\pm 0.3(\mathrm{syst})$. Under this hypothesis, the probability to observe six or more candidate events is $7\times {10}^{-3}$, equivalent to $2.5\sigma$ significance. At 90% C.L., the data are consistent with $0.03(0.04)<{sin}^{2}2{\theta }_{13}<0.28(0.34)$ for ${\delta }_{CP}=0$ and a normal (inverted) hierarchy.

The T2K experiment is a long baseline neutrino oscillation experiment. Its main goal is to measure the last unknown lepton sector mixing angle θ13θ13 by observing νeνe appearance in a νμνμ beam. It also aims to make a precision measurement of the known oscillation parameters, View the MathML sourceΔm232 and sin22θ23sin22θ23, via νμνμ disappearance studies. Other goals of the experiment include various neutrino cross-section measurements and sterile neutrino searches. The experiment uses an intense proton beam generated by the J-PARC accelerator in Tokai, Japan, and is composed of a neutrino beamline, a near detector complex (ND280), and a far detector (Super-Kamiokande) located 295 km away from J-PARC. This paper provides a comprehensive review of the instrumentation aspect of the T2K experiment and a summary of the vital information for each subsystem

The T2K Collaboration reports evidence for electron neutrino appearance at the atmospheric mass splitting, $|\Delta m_{32}^2|\approx 2.4\times {10}^{-3}{\mathrm{eV}}^2$. An excess of electron neutrino interactions over background is observed from a muon neutrino beam with a peak energy of 0.6 GeV at the Super-Kamiokande (SK) detector 295 km from the beam’s origin. Signal and background predictions are constrained by data from near detectors located 280 m from the neutrino production target. We observe 11 electron neutrino candidate events at the SK detector when a background of $3.3\pm 0.4(\mathrm{syst})$ events is expected. The background-only hypothesis is rejected with a $p$ value of 0.0009 ($3.1\sigma$), and a fit assuming ${\nu }_{\mu }\to {\nu }_e$ oscillations with ${sin}^{2}2{\theta }_{23}=1$, ${\delta }_{CP}=0$ and $|\Delta m_{32}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$ yields ${sin}^{2}2{\theta }_{13}={0.088}_{-0.039}^{+0.049}(\mathrm{stat}+\mathrm{syst})$.

T2K has performed the first measurement of ${\nu }_{\mu }$ inclusive charged current interactions on carbon at neutrino energies of $\sim 1\mathrm{GeV}$ where the measurement is reported as a flux-averaged double differential cross section in muon momentum and angle. The flux is predicted by the beam Monte Carlo and external data, including the results from the NA61/SHINE experiment. The data used for this measurement were taken in 2010 and 2011, with a total of $10.8\times {10}^{19}$ protons-on-target. The analysis is performed on 4485 inclusive charged current interaction candidates selected in the most upstream fine-grained scintillator detector of the near detector. The flux-averaged total cross section is $⟨{\sigma }_{\mathrm{CC}}{⟩}_{\varphi }=(6.91\pm 0.13(\mathrm{stat})\pm 0.84(\mathrm{syst}))\times {10}^{-39}\frac{{\mathrm{cm}}^2}{\mathrm{nucleon}}$ for a mean neutrino energy of 0.85 GeV.