Study of high energy atmospheric and astrophysical neutrinos by a large water Cherenkov detector

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1) We have established a new method to reconstruct visible energy of fully-contained (FC) events at SK by using PMT pulse shape information recorded by FADC. We applied this method to SK2 FC atmospheric neutrino sample and found conventional visible energy using ADC was underestimated by about factor 2 at most because of overflow of ADC.2) We have made an enriched sample of charged current nue interactions in SK2 FC atmospheric neutrino sample by applying several cuts and obtained a correct energy distribution of atmospheric nue based on FADC data. This is the first measurement of energy spectrum of atmospheric nue in the multi-10th energy region after the FREJUS experiment did at below 10 GeV. No event was found in more than 100 GeV region. It is consistent with 0.3 expected from atmospheric nue flux calculation.3) These results shown above was summarized in a master thesis of Nagoya University (T. Koike 2006) and reported in JPS meetings in autumn 06 and spring 07.4) Calibration work of FADC for SK2 and SK3 was done. Analysis program and MC codes for FADC were improved5) Design study of improved FADC trigger for up-going muon candidates. We leaned it requires substantial change in DAQ electronics of the outer detector of Super-Kamiokande.6) Linearity of 20-inch PMT used in the SK inner detector at very high photon input was tested by using a very fast timing pulsed N2 laser system. We observe it shows good linearity up to 700 p.e. at input.7) Neutrinos from the largest flare of SGR1806-20 at Dec27, 2005 were searched and reported in ICRC2005. We also have studied astrophysical neutrinos by using up-going muon events and published papers in refereed journals and conference proceedings. Also we have studied identification of nutau charged current events and nue charged current events related to this work. They were also published.