Abstract
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: Normal Hierarchy: $\sin^2\theta_{23}=0.514^{+0.055}_{-0.056}$ and $\Delta m^2_{32}=(2.51\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ Inverted Hierarchy: $\sin^2\theta_{23}=0.511\pm0.055$ and $\Delta m^2_{13}=(2.48\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ The analysis accounts for multi-nucleon 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 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% 1D credible intervals for the other oscillation parameters, when reactor data are included, are: $\sin^2\theta_{23}=0.528^{+0.055}_{-0.038}$ and $|\Delta m^2_{32}|=(2.51\pm0.11)\times 10^{-3}$ eV$^2$/c$^4$.
Highlights
Neutrino oscillation was firmly established in the late 1990s with the observation by the Super-Kamiokande (SK) experiment that muon neutrinos produced by cosmic ray interactions in our atmosphere changed their flavor [1]
This section describes the interaction model in NEUT, the primary neutrino interaction generator used by T2K, explains how we use data from external experiments to provide initial constraints on the model before fitting to T2K data, discusses remaining uncertainties not constrained by external data sources, and discusses uncertainties based on differences between the NEUT model and those found in other interaction generators
For all run periods with the horns operated at 250 kA, the neutrino event rate is found to be stable within 2% and the rms/mean of the event rate is 0.7%
Summary
Neutrino oscillation was firmly established in the late 1990s with the observation by the Super-Kamiokande (SK) experiment that muon neutrinos produced by cosmic ray interactions in our atmosphere changed their flavor [1]. This paper describes our most recent measurements of neutrino oscillation including our first results from analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance. With these data, almost 10% of the total proposed for the experiment, T2K enters the era of precision neutrino oscillation measurements. We summarize the near detectors and explain how we use their data to improve model predictions of neutrino interactions at the far detector. The final sections give detailed descriptions and results for the analysis of νμ disappearance alone [10] and for the joint analyses of νμ disappearance and νe appearance
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