Showing papers by "Tsuyoshi Nakaya published in 2013"

T2K neutrino flux prediction

Abstract: 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 Japan Proton Accelerator Research Complex 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—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 and muons that produce neutrinos. The simulation uses proton beam monitor measurements as inputs. The modeling of hadronic interactions is reweighted 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.

Evidence of electron neutrino appearance in a muon neutrino beam

Abstract: The T2K Collaboration reports evidence for electron neutrino appearance at the atmospheric mass splitting, | Δ m 2 32 | ≈ 2.4 × 10 − 3 eV 2 . An excess of electron neutrino interactions over background is observed from a muon neutrino beam with a peak energy of 0.6 GeV at the Super-Kamiokande (SK) detector 295 km from the beam’s origin. Signal and background predictions are constrained by data from near detectors located 280 m from the neutrino production target. We observe 11 electron neutrino candidate events at the SK detector when a background of 3.3 ± 0.4 ( syst ) events is expected. The background-only hypothesis is rejected with a p value of 0.0009 ( 3.1 σ ), and a fit assuming ν μ → ν e oscillations with sin 2 2 θ 23 = 1 , δ C P = 0 and | Δ m 2 32 | = 2.4 × 10 − 3 eV 2 yields sin 2 2 θ 13 = 0.088 + 0.049 − 0.039 ( stat + syst ) .

Measurement of neutrino oscillation parameters from muon neutrino disappearance with an off-axis beam

Abstract: The T2K collaboration reports a precision measurement of muon neutrino disappearance with an off-axis neutrino beam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrino flux and cross section parameters. The Super-Kamiokande far detector, which is 295 km downstream of the neutrino production target, collected data corresponding to 3.01×1020 protons on target. In the absence of neutrino oscillations, 205±17 (syst.) events are expected to be detected and only 58 muon neutrino event candidates are observed. A fit to the neutrino rate and energy spectrum assuming three neutrino flavors, normal mass hierarchy and θ23≤π/4 yields a best-fit mixing angle sin2(2θ23)=1.000 and mass splitting |Δm232|=2.44×10−3 eV2/c4. If θ23≥π/4 is assumed, the best-fit mixing angle changes to sin2(2θ23)=0.999 and the mass splitting remains unchanged.

Evidence for the appearance of atmospheric tau neutrinos in super-Kamiokande

Abstract: Super-Kamiokande atmospheric neutrino data were fit with an unbinned maximum likelihood method to search for the appearance of tau leptons resulting from the interactions of oscillation-generated tau neutrinos in the detector. Relative to the expectation of unity, the tau normalization is found to be 1.42 ± 0.35(stat)(-0.12)(+0.14)(syst) excluding the no-tau-appearance hypothesis, for which the normalization would be zero, at the 3.8σ level. We estimate that 180.1 ± 44.3(stat)(-15.2)(+17.8) (syst) tau leptons were produced in the 22.5 kton fiducial volume of the detector by tau neutrinos during the 2806 day running period. In future analyses, this large sample of selected tau events will allow the study of charged current tau neutrino interaction physics with oscillation produced tau neutrinos.

Measurement of the inclusive νμ charged current cross section on carbon in the near detector of the T2K experiment

Abstract: We thank the J-PARC accelerator team for the superb accelerator performance and CERN NA61 colleagues for providing essential particle production data and for their fruitful collaboration. We acknowledge the support of MEXT, Japan; NSERC, NRC and CFI, Canada; CEA and CNRS/IN2P3, France; DFG, Germany; INFN, Italy; Ministry of Science and Higher Education, Poland; RAS, RFBR and the Ministry of Education and Science of the Russian Federation; MEST and NRF, South Korea; MICINN and CPAN, Spain; SNSF and SER, Switzerland; STFC, UK; NSF and DOE, USA. We also thank CERN for their donation of the UA1/NOMAD magnet and DESY for the HERA-B magnet mover system. In addition, participation of individual researchers and institutions in T2K has been further supported by funds from: ERC (FP7), EU; JSPS, Japan; Royal Society, UK; DOE Early Career program, and the A. P. Sloan Foundation, USA.

Pion emission from the T2K replica target: Method, results and application

Abstract: The T2K long-baseline neutrino oscillation experiment in Japan needs precise predictions of the initial neutrino flux. The highest precision can be reached based on detailed measurements of hadron emission from the same target as used by T2K exposed to a proton beam of the same kinetic energy of 30 GeV. The corresponding data were recorded in 2007-2010 by the NA61/SHINE experiment at the CERN SPS using a replica of the T2K graphite target. In this paper details of the experiment, data taking, data analysis method and results from the 2007 pilot run are presented. Furthermore, the application of the NA61/SHINE measurements to the predictions of the T2K initial neutrino flux is described and discussed.

New Results from T2K

Abstract: We report new results from the T2K neutrino oscillation experiment with additional data collected after recovering from damage by the Great East Japan Earthquake of March 11, 2011. With the exposure of 2.56 × 10 20 protons on target, we observe 10 electron neutrino events in a muon neutrino beam with 2.73 ± 0.37 background events estimated for θ 13 = 0 , which constitutes the evidence of electron neutrino appearance and θ 13 ≠ 0 . The observation corresponds to 3.2σ excess ( p-value = 0.08 % ), and results in sin 2 2 θ 13 = 0.104 − 0.045 + 0.060 ( 0.128 − 0.075 + 0.070 ) for δ CP = 0 and Δ m 32 2 = 2.4 × 10 − 3 eV 2 for normal (inverted) mass hierarchy. This discovery opens the window to explore CP violation in the neutrino sector.

The development of a hybrid photo-detector (HPD) for the Hyper-Kamiokande project

Abstract: We are developing hybrid photo-detectors (HPDs) for Hyper-Kamiokande, which is a next generation underground water Cherenkov detector designed to search for proton decay and study many topics related to neutrino physics and astrophysics. A verification study of the suitability of HPDs for a water Cherenkov detector in a 200-ton tank is planned. In advance, we evaluated the performances of an 8-in. prototype HPD and tested them in water to confirm their safe operation. The prototype shows better single photon sensitivity and timing resolution than the conventional PMTs used in Super-Kamiokande.

Development and measurement of new large-aperture photodetectors for Hyper-Kamiokande

Abstract: Two types of new photodetectors are currently being developed with a large aperture, for use in the large water Cherenkov detector, Hyper-Kamiokande planned in Japan [1]: Hybrid Photo-Detector (HPD) of a 20-inch diameter size with an avalanche diode and a 20-inch Photomultiplier Tube (PMT) with a dynode upgrade to a box-and-line type These are expected to have an excellent performance about timing and collection efficiency compared to a 20-inch PMT with a venetian blind dynode used in Super-Kamiokande Also there is a possibility to improve quantum efficiency (QE) of these photodetectors from 22% to around 30% using the super-bialkali technique A proof test in a smaller water Cherenkov detector is planned in advance for these photodetectors to determine the best candidate for Hyper- Kamiokande Two prototypes of new photodetectors have already been manufactured for testing: the 8-inch HPD, and a 20-inch PMT with a higher QE of 30% compared with 22% of the 20-inch PMT used in Super-Kamiokande These photodetectors were installed in a 200-ton water tank in summer 2013 for a proof of using test in a water Cherenkov detector We report the specification of these photodetectors, a status and plan of the development and proof test