We present an analysis of atmospheric neutrino data from a 33.0 kton yr (535-day) exposure of the Super-Kamiokande detector. The data exhibit a zenith angle dependent deficit of muon neutrinos which is inconsistent with expectations based on calculations of the atmospheric neutrino flux. Experimental biases and uncertainties in the prediction of neutrino fluxes and cross sections are unable to explain our observation. The data are consistent, however, with two-flavor n m $ n t oscillations with sin 2 2u . Atmospheric neutrinos are produced as decay products in hadronic showers resulting from collisions of cosmic rays with nuclei in the upper atmosphere. Production of electron and muon neutrinos is dominated by the processes p 1 ! m 1 1 n m followed by m 1 ! e 1 1 n m 1 n e (and their charge conjugates) giving an expected ratio 1562 0031-9007͞98͞81(8)͞1562(6)$15.00
Solar neutrino measurements from 1258 days of data from the Super-Kamiokande detector are presented [? ]. The measurements are based on recoil electrons in the energy range 5.0-20.0 MeV. The measured solar neutrino flux is 2.32 ± 0.03 (stat.) +0.08 −0.07 (sys.) ×10 6 cm −2 s −1 , which is 45.1 ± 0.5 (stat.) +1.6 −1.4 (sys.)% of that predicted by the BP2000 SSM. The day vs night flux asymmetry (Φn − Φ d )/Φaverage is 0.033 ± 0.022 (stat.) +0.013 −0.012 (sys.). The recoil electron energy spectrum is consistent with no spectral distortion (χ 2 /d.o.f. = 19.0/18). The seasonal variation of the flux is consistent with that expected from the eccentricity of the Earth's orbit (χ 2 /d.o.f. = 3.7/7). For the hep neutrino flux, we set a 90% C.L. upper limit of 40 × 10 3 cm −2 s −1 , which is 4.3 times the BP2000 SSM prediction.22 This preprint is almost identical to the report submitted to Physical Review Letter. We have added to this preprint a few tables of
We propose the Hyper-Kamiokande (Hyper-K) detector as a next generation underground water Cherenkov detector. It will serve as a far detector of a long baseline neutrino oscillation experiment envisioned for the upgraded J-PARC, and as a detector capable of observing -far beyond the sensitivity of the Super-Kamiokande (Super-K) detector -proton decays, atmospheric neutrinos, and neutrinos from astronomical origins. The baseline design of Hyper-K is based on the highly successful Super-K, taking full advantage of a well-proven technology.Hyper-K consists of two cylindrical tanks lying side-by-side, the outer dimensions of each tank being 48 (W) × 54 (H) × 250 (L) m 3 . The total (fiducial) mass of the detector is 0.99 (0.56) million metric tons, which is about 20 (25) times larger than that of Super-K. A proposed location for Hyper-K is about 8 km south of Super-K (and 295 km away from J-PARC) at an underground depth of 1,750 meters water equivalent (m.w.e.). The inner detector region of the Hyper-K detector is viewed by 99,000 20-inch PMTs, corresponding to the PMT density of 20% photo-cathode coverage (one half of that of Super-K).Hyper-K presents unprecedented potential for precision measurements of neutrino oscillation parameters and discovery reach for CP violation in the lepton sector. With a total exposure of 5 years (one year being equal to 10 7 sec) to a 2.5-degree off-axis neutrino beam produced by the 1.66 MW J-PARC proton synchrotron, it is expected that the CP phase δ can be determined to better than 18 degrees for all possible values of δ and CP violation can be established with a statistical significance of 3σ for 74% of the δ parameter space if sin 2 2θ 13 > 0.03 and the mass hierarchy is known. If sin 2 2θ 13 is as large as 0.1 the mass hierarchy can be determined with more than 3σ statistical significance for 46% of the δ parameter space. In addition, a high statistics data sample of atmospheric neutrinos will allow us to extract the information on the mass hierarchy and the octant of θ 23 . With a full 10 year duration of data taking, the significance for the mass hierarchy determination is expected to reach 3σ or greater if sin 2 θ 23 > 0.4.Hyper-K can extend the sensitivity to nucleon decays beyond what was achieved by Super-K by an order of magnitude or more. The sensitivities to the partial lifetime of protons for the decay modes of p → e + π 0 and p → νK + are expected to exceed 1 × 10 35 years and 2 × 10 34 years, respectively. This is the only known, realistic detector option capable of reaching such a sensitivity for the p → e + π 0 mode.The scope of studies at Hyper-K also covers high precision measurements of solar neutrinos, observation of both supernova burst neutrinos and supernova relic neutrinos, dark matter searches, and possible detection of solar flare neutrinos. The prospects for neutrino geophysics using Hyper-K are also mentioned.
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