Highlights from T2K

T2K is a long baseline neutrino oscillation experiment. A high intensity proton beam at J-PARC produces a narrow-band muon-neutrino beam with a peak energy of 0.6 GeV at the far detector, Super-Kamiokande, 295km from the production point. The T2K experiment has observed electron neutrino appearance in a muon neutrino beam and measured sin2θ13 = 0.140+0.038 −0.032 (0.170 +0.045 −0.037) assuming the normal (inverted) hierarchy. In addition the latest measurements of θ23 and Δm32 are reported, along with the latest T2K neutrino interaction cross section measurements of the inclusive CC, CCQE and NCE channels.


Introduction
In the standard 3 neutrino model, the flavour states are related to the mass states through the PMNS matrix [1] [2]. This mixing matrix is conventionally parameterised by three real mixing angles (θ 12 , θ 23 , θ 13 ) and one complex CP violating phase (δ CP ). The oscillation probability also depends on the neutrino mass splittings, Δm 2 i j = m 2 i − m 2 j . In the T2K experiment [3] a mostly ν μ beam with a peak energy of 0.6 GeV is directed towards the Super-Kamiokande detector at a distance of 295 km from the source. The appearance channel (ν μ → ν e ) is sensitive to θ 13 . The disappearance channel (ν μ → ν μ ) is sensitive to θ 23 and Δm 2 23 . This note reports the observation of the ν μ → ν e channel and the consequent measurement of θ 13 . The measurement of non-zero θ 13 opens the way to measurement of δ CP . In addition, the latest results from the ν μ → ν μ channel and neutrino interaction cross section measurements are reported.

The T2K Experiment
A high intensity 30 GeV proton beam at J-PARC is directed towards a graphite target. The charged hadrons produced are focussed by magnetic horns to produce a mostly ν μ beam. The far detector is located 2.5 • degrees off-axis. This exposes the far detector to a narrow-band beam peaked at 0.6 GeV which is optimised to give the maximum appearance probability and the minimum background to the ν e appearance analysis. The predicted flux at the ND280 near detector is shown in fig. 1. The beam prediction is based on simulations tuned to external hadron production data from the NA61/SHINE experiment [4] [5] and in-situ measurements from muon monitors. A total of 6.6 × 10 20 POT was delivered up to 2013.
The near detector complex at 280m from the target consists of two main detectors, IN-GRID (on-axis) and ND280 (off-axis), providing a range of target materials and detector technologies. Their purpose is two-fold. First to directly measure the neutrino beam. Second to measure neutrino interaction cross sections.
INGRID [6] consists of an array of 16 iron/scintillator sandwich modules and a single fine grained pure scintillator detector. The modules cover a range of x − y positions allowing measurement of the profile of the neutrino beam. As well as monitoring the neutrino beam INGRID will also be used to measure neutrino cross sections.
The off-axis near detector, shown in fig. 2, is divided into a Tracker and π 0 detector region. The Fine Grained Detector [7] (FGD1) consists of layers of plastic scintillator bars, 10×10 mm in cross section, read out with wavelength shifting fibers into Multi-Pixel Photon Counters (MPPCs). It provides target mass and track reconstruction near the interaction vertex. The Time Projection Chambers [8] (TPCs) provide PID based on dE/dx in the Argon based gas and momentum measurement from track curvature in the magnetic field.
The π 0 detector [9] (P0D) consists of alternating layers of plastic scintillator bars 1 , water bags and brass/lead sheets surrounded by e. m. calorimeters. Its main goal is to measure interactions producing π 0 in the final state.
Charged current ν μ interactions are selected by requiring a reconstructed muon originating in the FGD fiducial volume. Final state charged pions are identified by late charge deposits from decay electrons or additional reconstructed tracks with dE/dx consistent with a pion. The event sample is divided into three topologies based on the number of pions reconstructed in the final state: CC0π, CC1π and other. The reconstructed muon momentum is shown in fig. 3. The prior beam flux and neutrino interaction model is fitted to the observed muon p − cos(θ) distribution of these samples. This choice of topologies allows the data to constrain the parameters of the interaction model governing quasi-elastic scattering and resonant pion production. The post-fit model with reduced total uncertainty is propogated to the far detector oscillation analyses.
The far detector, Super-Kamiokande (SK), shown in fig. 4, is a 50kt Water Cherenkov detector located 1km underground in the Mozumi Mine in Kamioka. The detector contains ∼ 13, 000 PMTs that image the neutrino interactions. Analysis of the reconstructed Cherenkov rings allows determination of charged lepton flavour and hence the initial neutrino flavour.

ν e Appearance
Fully contained, electron-like single ring events are selected. Events with late time signals or electron momentum less than 100 MeV are rejected to remove decay electrons from μ and π. Events with reconstructed neutrino energy E reco > 1250 MeV are rejected as these are mostly intrinsic beam background.
A new PID algorithm improves the π 0 rejection compared to previous T2K analyses. For each PMT, the expected hit charge and time PDF for a given particle species, position and momentum is computed. A combined likelihood is formed from the product over all PMTs and all particles in the case of multiple particle final states such as π → γγ. A maximum likelihood fit is performed for each particle hypothesis. The reconstructed position and momentum is taken from the best-fit parameters. The minimized likelihood value is used to determine the particle species. π 0 background is rejected by cutting on the reconstruced γγ invariant mass and the minimized log likelihood ratio ln(L π 0 /L e ).
The selected events are shown in fig. 5. A total of 28 ν e candidate events are selected. The expected background is 4.92 ± 0.55 events. These results are based on 6.6 × 10 20 POT.

ν μ Disappearance
A similar analysis is performed on the disappearance channel to measure sin 2 (θ 23 ) and Δm 2 32 . Fully contained, muon-like single ring events with visible energy > 30 MeV, reconstructed muon momentum > 200 MeV and ≤ 1 decay electron are selected.
The selected events are shown in fig. 7. 58 candidate ν μ events are observed. With no oscillation 205 ± 17 events are expected. These results are based on 3.0 × 10 20 POT.

Cross Section Measurements
Neutrino interactions on nuclei form the signal in all current and planned future long-baseline neutrino oscillation experiments. Understanding these interactions in the energy regime covered by the T2K beam flux is essential for the study of neutrino oscillations. The latest T2K cross section measurements in Charged Current (CC) Inclusive, CC quasi-elastic scattering (CCQE) and Neutral Currant Elastic (NCE) are shown in the following sections. These three measurements are based on the ND280 detector described in section 2.

CC Inclusive Cross Section Measurement
ν μ CC interactions are selected by requiring events with a reconstructed μ − starting within the FGD fiducial volume. Tracks are required to be negative, forward-going and have a muonlike TPC PID. This event selection achieves an efficiency of 50% and a purity of 87%. The dominant background is from out of fiducial volume interactions.
The cross section is extracted from the reconstructed p μ − cos(θ μ ) distribution. The NC background is subtracted and an efficiency correction is applied based on predictions from MC. An iterative Bayesian unfolding method is used to correct for reconstruction bias and unsmear distributions.

CCQE Cross Section
The Charged-Current Quasi-Elastic (CCQE) interaction, ν + n → l − + p, is the main signal in the neutrino oscillation measurement. This two-body interaction is important because the initial neutrino energy can be inferred from final state lepton kinematics. Additional cuts are applied to the CC inclusive sample to select interactions containing a muon with no pions in the final state. This event selection achieves an efficiency of 40% and purity 72% for true CCQE events.
The CCQE cross section is extracted by fitting the NEUT MC model to the reconstructed p μ − cos(θ μ ) distribution. This is shown in Figure 10 [13]. Systematic uncertainties are accounted for by varying bin contents with nuisance parameters. A maximum likelihood fit is used to find the best fit parameters. A χ 2 test comparing the fitted result with the nominal NEUT model gives p-value of 17% indicating agreement between the data and the cross section model.
An alternative approach to fitting the cross-section normalisation, is to directly fit the model parameters. The axial mass parameter M QE A is varied to obtain the best fit to the observed data. The fit is performed once using both shape and normalisation and again using only shape information.

Neutral Current Elastic Cross Section Measurement
The Neutral Current Elastic channel is ν μ + p(n) → ν μ + p(n). The observable signal produced in the detector is a proton track. Note that cases where the target nucleon is a neutron can also produce protons due to final state interactions in the nucleus. In the T2K analysis the signal is defined as all NC interactions with only nucleons leaving the nucleus (no charged leptons or pions).
There are two main backgrounds to this channel. First, charged current interactions where the μ and p tracks are back-to-back and hence are reconstructed as the same track. This background is removed with PID based on dE/dx along the reconstructed track. Second, background from external interactions. This is measured in-situ from side-band samples. 3936 events are selected with an expect background of 2016. The flux integrated absolute cross section is calculated as σ flux = 2.24 ± 0.07(stat) +0.53 −0.63 (syst) × 10 −39 cm 2 [14]. This can be compared with MC predictions from NEUT (GENIE) of σ flux = 2.02(1.79) × 10 −39 cm 2 This analysis is based on 9.9 × 10 19 POT of water-in data. Future analyses will use water-out data to extract the cross-section on water.

Conclusion and Outlook
The T2K experiment has recorded 6.6 × 10 20 POT recorded to date. With only a fraction of the approved POT, the T2K experiment has discovered ν e appearance and is producing world leading measurements of the θ 23 mixing angle. T2K will continue to improve the neutrino oscillation measurements and produce further neutrino interaction cross section measurements. In 2014 T2K will run for the first time in anti-neutrino mode. The observation of non-zero θ 13 allows the measurement of δ CP . The T2K experiment will lead the search for leptonic CP violation in the coming years.       Figures (a-d) show the μ − momentum distribution in each bin of cos(θ μ ). The ND280 result is shown along side the prediction from the NEUT and GENIE neutrino event generators.