M. Nakahata

Atmospheric Neutrino Background and Pion Nuclear Effect for KAMIOKA Nucleon Decay Experiment

The main source of background for nucleon decay experiments is the interaction of atmospheric neutrinos in the detector. To estimate this background a Monte Carlo program was developed, which simulates the neutrino interactions and the nuclear effects of secondary pions. The program reproduced existing neutrino data as well as global features of the contained events in the KAMIOKA nucleon decay experiment. Neutrino induced background for nucleon decay was then calculated.

Search for Nucleon Decay Into Charged Lepton + Mesons

With a 3000 ton water Cerenkov detector operated 2700 m.w.e. underground, 103 fully contained events were observed during a live time of 343 days. Most of the events are well interpreted as due to ν interactions. Four multi-ring events survive after applying criteria for nucleon decay. The lower limits on τ/ B obtained from these data exceed 10 31 yr (90% C.L.) for most of the possible decay modes.

Search for Nucleon Decays Catalyzed by Magnetic Monopoles

Search for nucleon decays induced by magnetic monopoles has been carried out with a 3000 ton water Cherenkov detector by two independent methods. Search for multiple nucleon decays in the detector gives the cross section and velocity dependent limits on the monopole flux, F M <2.5×10 -15 cm -2 s - sr -1 for 5×10 -5 <β M <10 -3 and for σ=100 mb at 90% C.L. Search for low energy neutrinos (30< E ν <52 MeV) from the Sun gives a limit, F M (σ 0 /1 mb)<1.6×10 -21 (β M /10 -3 ) 2 cm -2 s -1 sr -1 at 90% C.L.

Search for Nucleon Decays Into Anti-neutrino + Mesons

With a 3000 ton water Cherenkov detector, the nucleon decays into an antineutrino+mesons are searched for. No evidence for nucleon decay has been found during 474 days of detector live time. The lower limits on partial nucleon life time, τ/ B , are 0.5-4.2×10 31 yr at 90% C.L. depending on the nucleon decay modes.

Time Variation of the Cosmic Ray Muon Flux in Underground Detectors and Correlation with Atmospheric Temperature

Time variations of the cosmic ray muon flux observed during the period from January 1987 to April 1990 in the Matsushiro underground muon observatory and the Kamiokande-II detector are compared. A significant correlation between the two observations is found. The observed variations are well explained as a temperature effect in the atmosphere based on precise atmospheric temperature measurements by the Wajima Observatory of the Japan Meteorological Agency.

A Test of New Large Aperture Photodetectors in a Water Cherenkov Detector

Yuji OKAJIMAa, Miao JIANGb, Daisuke FUKUDAc, Ryosuke AKUTSU, Yusuke SUDAd, Seiko HIROTAb, Shoei NAKAYAMAe, Masato SHIOZAWAe, Masashi YOKOYAMAd, Yoshinari HAYATOe, Hidekazu TANAKAe, Masayuki NAKAHATAe, Tsuyoshi NAKAYAb, Masahiro KUZEa, Yusuke KOSHIOc, Akimichi TAKETA f aDepartment of Physics, Tokyo Institute of Technology bDepartment of Physics, Kyoto University cDepartment of Physics, Okayama University dDepartment of Physics, The University of Tokyo eKamioka Observatory, Institute for Cosmic Ray Research, The University of Tokyo f Earthquake Research Institute, The University of Tokyo

Detailed performance evaluation of a new 20-inch photomultiplier tube with a Box and Line dynode

Yasuhiro Nishimuraa, Ryosuke Akutsua, Yusuke Sudab, Miao Jiangc, Seiko Hirotac, Daisuke Fukudad , Masahiro Kuzee, Masaki Ishitsukae, Masayuki Nakahataa, Masato Shiozawaa, Yoshinari Hayatoa, Shoei Nakayamaa, Hidekazu Tanakaa, Masashi Yokoyamab, Tsuyoshi Nakayac, Akihiro Minaminoc, Akimichi Taketa f , Yoshihiko Kawaig, Takayuki Ohmurag, Masatoshi Suzukig a Institute for Cosmic Ray Research, University of Tokyo b Department of Physics, University of Tokyo c Department of Physics, Kyoto University d Department of Physics, Okayama University e Department of Physics, Tokyo Institute of Technology f Earthquake Research Institute, University of Tokyo g Hamamatsu Photonics K.K.

Temperature Dependence Measurement of a Hybrid Photo-Detector for Hyper-Kamiokande

1 Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan 2 Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan 3 Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan 4 Department of Physics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan 5 Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo 113-0032, Japan 6 Hamamatsu Photonics K.K., Hamamatsu, Shizuoka, 430-8587, Japan

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

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.

Astroparticle physics with solar neutrinos

Solar neutrino experiments observed fluxes smaller than the expectations from the standard solar model. This discrepancy is known as the “solar neutrino problem”. Flux measurements by Super-Kamiokande and SNO have demonstrated that the solar neutrino problem is due to neutrino oscillations. Combining the results of all solar neutrino experiments, parameters for solar neutrino oscillations are obtained. Correcting for the effect of neutrino oscillations, the observed neutrino fluxes are consistent with the prediction from the standard solar model. In this article, results of solar neutrino experiments are reviewed with detailed descriptions of what Kamiokande and Super-Kamiokande have contributed to the history of astroparticle physics with solar neutrino measurements.