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Takashi Kobayashi: Results of T2K

March 14, 2013

T2k consists of 500 collaborators from all around the world. T2K (Tokai to Kamioka) uses a muon neutrino beam from J-PARC MR to superkamiokande at 295km.

The importance of measuring electron neutrino appearance as a next critical step toward CP measurement was by Nishikawa &Totsuka in 1999. T2K is one experiment addressing that experimental challenge. Data started to be collected in 2010. The experiment uses the 30 GeV proton beam from J-Parc MR, off-axis from the far detector direction. The peak intensity is at 0.6 GeV.  There is also a near detector at 280m.

The far detector is Super-Kamiokande. A 50kton water cherenkov detector operational since 1996. The data taking in the T2K setup started in 2010 and so far obtained 5.63×10^20 POT, which is just 7% of the total approved POT budget. Stable operation at 230 kW corresponds to 10^14 protons per pulse. New results on muon neutrino disappearance (feb 2013) is based on data collected since 2011.

The speaker discussed beam stability issues, monitored with muons and by the neutrino event rate measured by INGRID. Also the neutrino direction is measured and found within the requirements.

The analysis strategy foresees a precise prediction of SK fluxes based on ND280 and NA61 measurements. The comparison of observed and predicted yields allow to measure excesses and constrain oscillation parameters.

The flux extrapolations are sensitive to the parent hadron production, their momentum and angle distributions. A Geant3 simulation is used, pion and kaon production is tuned to experimental data from NA61, and a normalization in bins of neutrino energy and flavour is performed. The total error amounts to 15%.

The near detector (ND280) allows a constraint on flux times cross section of neutrinos. THe data is divided in two samples binned in muon momentum and angle: the CCQE and CCnQE (charged current quasi-elastic and non-quasi-elastic events). The agreement in the predicted and observed spectra is very good.

The impact of these flux measurements on the predicted number of electron neutrino appearance events in SuperKamiokande varies between 10 and 13%, driven by several factors of similar relative importance. On the prediction of muon neutrino disappearance the fractional error is largest at sub-GeV reconstructed energy, then flat at about 20%.

The new results of T2K on muon neutrino disappearance uses Run1-3 data, corresponding to 3×10^20 POT. Events at far detector are single-ring, muon-like event, with momentum greater than 200 MeV/c, and at most one decay electron identified. There are 58 event candidates, with good agreement with simulations. One can determine the values of theta_23 and dm^2_32.

The result is the world’s best precision on theta_23. The likelihood ratio contour at 90% CL encompasses the region above 0.94 in sin^2 2 theta_23 for dm^2 values between 2.1 and 2.8 eV^2. Systematics do not change very much the shape or extent of the allowed region.

On electron neutrino appearance, they find 11 candidate events, with 3.22+-0.43 for sin^2 2theta_13=0. The p-value is 0.08% (a 3.2 sigma effect). So T2K sees a first indication of non-zero theta_13, and now establishes an evidence of electron neutrino appearance from muon neutrinos. By the summer of 2013, they want to achieve a five-sigma effect on the electron neutrino appearance.

T2K also performed cross section measurement of neutral and charged-current processes. This helps reducing the systematic uncertainty on oscillation parameters.

The physics program of T2K in the near future includes precision measurements of both appearance and disappearance: first hints of CPV and mass hierarchy, dm_13, and searches for sterile neutrinos, plus various cross section measurements at the near detector.

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