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 θ 23 ¼ 0.514 þ0.055 −0.056 and Δm 2 32 ¼ ð2.51 AE 0.10Þ × 10 −3 eV 2 =c 4 and inverted hierarchy∶ sin 2 θ 23 ¼ 0.511 AE 0.055 and Δm 2 13 ¼ ð2.48 AE 0.10Þ × 10 −3 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, jΔm 2 j, sin 2 θ 23 , sin 2 θ 13 , δ 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 δ CP ¼ ½0.15; 0.83 π for normal hierarchy and δ CP ¼ ½−0.08; 1.09 π 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 θ 23 ¼ 0.528 þ0.055 −0.038 and jΔm 2 32 j ¼ ð2.51 AE 0.11Þ × 10 −3 eV 2 =c 4 .

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, PRL 112, 061802 (2014) P H Y S I C A L R E V I E W L E T T E R Sweek ending 14 FEBRUARY 2014 061802-2 corresponding to a significance of 7.3σ when compared to 4.92 AE 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 θ 12 , θ 23 , θ 13 , a mass difference Δm 2 32 and a CP violating phase δ CP . In this neutrino oscillation scenario, assuming jΔm 2 32 j ¼ 2.4 × 10 −3 eV 2 , sin 2 θ 23 ¼ 0.5, and Δm −0.037 ) is obtained at δ CP ¼ 0. When combining the result with the current best knowledge of oscillation parameters including the world average value of θ 13 from reactor experiments, some values of δ CP are disfavored at the 90% C.L. DOI: 10.1103/PhysRevLett.112.061802 PACS numbers: 14.60.Pq, 14.60.Lm, 25.30.Pt, 29.40.Ka Introduction.-The discovery of neutrino oscillations using atmospheric neutrinos was made by SuperKamiokande in 1998 [1]. Since then, many other experiments have confirmed the phenomenon of neutrino oscillations through various disappearance modes of flavor transformations. However, to date, there has not been an observation of the explicit appearance of a different neutrino flavor from neutrinos of another flavor through neutrino oscillations. In 2011, the T2K collaboration published the first indication of electron neutrino appearance from a muon neutrino beam at 2.5σ significance based on a data set corresponding to 1.43 × 10 20 protons on target (POT) [2,3]. This result was followed by the publication of further evidence for electron neutrino appearance at 3.1σ in early 2013 [4]. This Letter presents new results from the T2K experiment that establish, at greater than 5σ, the observation of electron-neutrino appearance from a muon-neutrino beam.In a three-flavor framework, neutrino oscillations are described by the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix [5,6] which is parametrized by three mixing angles θ 12 , θ 23 , θ 13 , and a CP violating phase δ CP . In this framework, the probability for ν μ → ν e oscillation can be expressed [7] as where L is the neutrino propagation distance and E is the neutrino energy. The measurement of ν μ → ν e oscillations is of particular interest because this mode is sensitive to both θ 13 and δ CP . The first indication of nonzero θ 13 was published by T2K [3] based on the measurement of ν μ → ν e oscillations. More recently, indications of ν μ → ν e oscillations were also reported by the MINOS experiment [8]. The value of θ 13 is now precisely known to be 9.1°AE 0.6°from measurements ofν e disappearance in reactor neutrino experiments [9][10][11][12]. Using the reactor measurement of θ 13 , the ν μ → ν e appearance mode can be used to ...

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 detectors 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 3 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.

We report the measurement of muon neutrino charged-current interactions on carbon without pions in the final state at the T2K beam energy using 5.734 × 10 20 protons on target. For the first time the measurement is reported as a flux-integrated, double-differential cross section in muon kinematic variables (cos θ μ , p μ ), without correcting for events where a pion is produced and then absorbed by final state interactions. Two analyses are performed with different selections, background evaluations and cross-section extraction methods to demonstrate the robustness of the results against biases due to model-dependent assumptions. The measurements compare favorably with recent models which include nucleon-nucleon correlations but, given the present precision, the measurement does not distinguish among the available models. The data also agree with Monte Carlo simulations which use effective parameters that are tuned to external data to describe the nuclear effects. The total cross section in the full phase space is σ ¼ ð0.417 AE 0.047ðsystÞ AE 0.005ðstatÞÞ × 10 −38 cm 2 nucleon −1 and the cross section integrated in the region of phase space with largest efficiency and best signal-over-background ratio (cos θ μ > 0.6 and p μ > 200 MeV) is σ ¼ ð0.202 AE 0.036ðsystÞ AE 0.003ðstatÞÞ × 10 −38 cm 2 nucleon −1 .

New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter θ23. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to 6.57 × 10 20 protons on target, T2K has fit the energydependent νµ oscillation probability to determine oscillation parameters. The 68% confidence limit 3 on sin 2 (θ23) is 0.514−0.056 (0.511 ± 0.055), assuming normal (inverted) mass hierarchy. The bestfit mass-squared splitting for normal hierarchy is ∆m 2 32 = (2.51 ± 0.10) ×10 −3 eV 2 /c 4 (inverted hierarchy: ∆m 2 13 = (2.48 ± 0.10) ×10 −3 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.