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 experiment has observed electron neutrino appearance in a muon neutrino beam produced 295 km from the Super-Kamiokande detector with a peak energy of 0.6 GeV. A total of 28 electron neutrino events were detected with an energy distribution consistent with an appearance signal, corresponding to a significance of $7.3\sigma$ when compared to $4.92\pm 0.55$ expected background events. In the Pontecorvo-Maki-Nakagawa-Sakata mixing model, the electron neutrino appearance signal depends on several parameters including three mixing angles ${\theta }_{12}$, ${\theta }_{23}$, ${\theta }_{13}$, a mass difference $\mathrm{\Delta }{m}_{32}^2$ and a $CP$ violating phase ${\delta }_{CP}$. In this neutrino oscillation scenario, assuming $|\mathrm{\Delta }{m}_{32}^2|=2.4\times {10}^{-3}{\mathrm{eV}}^2$, ${\sin }^2{\theta }_{23}=0.5$, and $\mathrm{\Delta }{m}_{32}^2>0$ ($\mathrm{\Delta }{m}_{32}^2<0$), a best-fit value of ${\sin }^{2}2{\theta }_{13}=0.140_{-0.032}^{+0.038}$ ($0.170_{-0.037}^{+0.045}$) is obtained at ${\delta }_{CP}=0$. When combining the result with the current best knowledge of oscillation parameters including the world average value of ${\theta }_{13}$ from reactor experiments, some values of ${\delta }_{CP}$ are disfavored at the 90% C.L.

We report on measurements of neutrino oscillation using data from the T2K long-baseline neutrino experiment collected between 2010 and 2013. In an analysis of muon neutrino disappearance alone, we find the following estimates and 68% confidence intervals for the two possible mass hierarchies: $\text{normal hierarchy}:{\sin }^2{\theta }_{23}=0.514_{-0.056}^{+0.055}$ and $\mathrm{\Delta }{m}_{32}^2=(2.51\pm 0.10)\times 10^{-3}{\mathrm{eV}}^2/c^4$ and $\text{inverted hierarchy}:{\sin }^2{\theta }_{23}=0.511\pm 0.055$ and $\mathrm{\Delta }{m}_{13}^2=(2.48\pm 0.10)\times 10^{-3}{\mathrm{eV}}^2/c^4$. The analysis accounts for multinucleon mechanisms in neutrino interactions which were found to introduce negligible bias. We describe our first analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance to estimate four oscillation parameters, $|\mathrm{\Delta }{m}^2|$, ${\sin }^2{\theta }_{23}$, ${\sin }^2{\theta }_{13}$, ${\delta }_{CP}$, and the mass hierarchy. Frequentist and Bayesian intervals are presented for combinations of these parameters, with and without including recent reactor measurements. At 90% confidence level and including reactor measurements, we exclude the region ${\delta }_{CP}=[0.15,0.83]\pi$ for normal hierarchy and ${\delta }_{CP}=[-0.08,1.09]\pi$ for inverted hierarchy. The T2K and reactor data weakly favor the normal hierarchy with a Bayes factor of 2.2. The most probable values and 68% one-dimensional credible intervals for the other oscillation parameters, when reactor data are included, are ${\sin }^2{\theta }_{23}=0.528_{-0.038}^{+0.055}$ and $|\mathrm{\Delta }{m}_{32}^2|=(2.51\pm 0.11)\times 10^{-3}{\mathrm{eV}}^2/c^4$.

New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter ${\theta }_{23}$. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to $6.57\times 10^{20}$ protons on target, T2K has fit the energy-dependent ${\nu }_{\mu }$ oscillation probability to determine oscillation parameters. The 68% confidence limit on ${\sin }^2({\theta }_{23})$ is $0.514_{-0.056}^{+0.055}$ ($0.511\pm 0.055$), assuming normal (inverted) mass hierarchy. The best-fit mass-squared splitting for normal hierarchy is $\mathrm{\Delta }{m}_{32}^2=(2.51\pm 0.10)\times {10}^{-3}{\mathrm{eV}}^2/c^4$ (inverted hierarchy: $\mathrm{\Delta }{m}_{13}^2=(2.48\pm 0.10)\times {10}^{-3}{\mathrm{eV}}^2/c^4$). Adding a model of multinucleon interactions that affect neutrino energy reconstruction is found to produce only small biases in neutrino oscillation parameter extraction at current levels of statistical uncertainty.