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.

The development and operation of liquid-argon time-projection chambers for neutrino physics has created a need for new approaches to pattern recognition in order to fully exploit the imaging capabilities offered by this technology. Whereas the human brain can excel at identifying features in the recorded events, it is a significant challenge to develop an automated, algorithmic solution. The Pandora Software Development Kit provides functionality to aid the design and implementation of pattern-recognition algorithms. It promotes the use of a multi-algorithm approach to pattern recognition, in which individual algorithms each address a specific task in a particular topology. Many tens of algorithms then carefully build up a picture of the event and, together, provide a robust automated pattern-recognition solution. This paper describes details of the chain of over one hundred Pandora algorithms and tools used to reconstruct cosmic-ray muon and neutrino events in the MicroBooNE detector. Metrics that assess the current pattern-recognition performance are presented for simulated MicroBooNE events, using a selection of final-state event topologies.