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

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A total of 614 upward through-going muons of minimum energy 1.6 GeV are observed by Super-Kamiokande during 537 detector live days. The measured muon flux is (1.74 ± 0.07(stat.) ± 0.02(sys.)) × 10 −13 cm −2 s −1 sr −1 compared to an expected flux of (1.97 ± 0.44(theo.)) × 10 −13 cm −2 s −1 sr −1 . The absolute measured flux is in agreement with the prediction within the errors. However, the zenith angle dependence of the observed upward through-going muon flux does not agree with no-oscillation predictions. The observed distortion in shape is consistent with the νµ ↔ ντ oscillation hypothesis with sin 2 2θ > 0.4 and 1 × 10 −3 < ∆m 2 < 1 × 10 −1 eV 2 at 90 % confidence level.

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A search for day-night variations in the solar neutrino flux resulting from neutrino oscillations has been carried out using the 504 day sample of solar neutrino data obtained at Super-Kamiokande. The absence of a significant day-night variation has set an absolute flux independent exclusion region in the two neutrino oscillation parameter space.14.60. Pq,26.65.+t,96.40.Tv,95.85.Ry Typeset using REVT E X 2 As a real time solar neutrino experiment, Super-Kamiokande can perform a wide range of time modulation studies of the solar neutrino flux. One motivation for these types of studies is an investigation of neutrino oscillation hypotheses. All solar neutrino observations [1][2][3][4][5] have reported significantly lower fluxes than the expectations of standard solar models (SSMs) [6][7][8]. This difference is commonly referred to as the solar neutrino problem. Given the support of recent helioseismological observations [9], these SSMs look well-established and reliable. The difference between observations and predictions suggests some neutrino properties beyond the standard model of elementary particles. The most popular solution to the solar neutrino problem is neutrino oscillations, aided by matter enhanced oscillations in the Sun [10]. In some regions of the parameter space for neutrino oscillations, matter enhanced oscillations within the Earth can lead to a regeneration of the measured neutrino flux passing through the Earth. This regeneration would produce a higher flux measured during night time relative to day time measurements. If such a day-night variation was observed, it would be strong evidence for neutrino oscillations. The amplitude of the daynight flux variation would determine the neutrino oscillation parameters, independent of the absolute flux uncertainties of the SSMs.Super-Kamiokande (SK) started taking data in April, 1996. SK has already confirmed the deficit of solar neutrinos [5]. In this report, the total live time is increased to 503.8 days (May 31 st , 1996 through March 25 th , 1998), and the total number of solar neutrino events found coming from the Sun is now 6823 +148 −130 events above a threshold of 6.5 MeV in total energy of the recoil electron. Day-night (DN) variations are investigated with this high-statistics solar neutrino data sample and an updated flux value is presented.The SK detector is located at the Kamioka Observatory, Institute for Cosmic Ray Research, the University of Tokyo, in Gifu Prefecture, Japan, 137.32 degrees East longitude and 36.43 degrees North latitude. Due to the latitude, the nadir of the Sun can range between ±0.974 in cosine (12.98 to 167.02 degrees) at the SK site. In this analysis, nadir (θ z ) is defined as the angle between the negative z-axis of the detector coordinate system and the direction to the Sun (solar neutrino direction), where the cosine of the nadir is positive when the Sun is below the horizon. Solar neutrinos will penetrate different regions of the Earth, that are related to the nadir of the Sun by the following: mantle...