1999 Fiscal Year Final Research Report Summary
Precise measurement of the zenith-angle distribution of atomospheric neurtrinos and a study of neutrino oscillations
| Project/Area Number |
09440101
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
素粒子・核・宇宙線
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| Research Institution | The University of Tokyo |
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Principal Investigator |
KAJITA Takaaki ICRR, The Univ. of Tokyo, Professor, 宇宙線研究所, 教授 (40185773)
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| Co-Investigator(Kenkyū-buntansha) |
SHIOZAWA Masato ICRR, The Univ. of Tokyo, Research Associate, 宇宙線研究所, 助手 (70272523)
ITOW Yoshitaka ICRR, The Univ. of Tokyo, Research Associate, 宇宙線研究所, 助手 (50272521)
ANEYUKI Kenji ICRR, The Univ. of Tokyo, Associate Professor, 宇宙線研究所, 助教授 (30224629)
OKADA Atsushi ICRR, The Univ. of Tokyo, Research Associate, 宇宙線研究所, 助手 (90013341)
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| Project Period (FY) |
1997 – 1999
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| Keywords | atmospheric neutrino / neutrino / neutrino oscillation / mu-neutrino / tau-neutrino |
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| Research Abstract |
We studied neutrino oscillations using the atmospheric neutrino data from Super Kamiokande.
Multi-GeV muon-like data showed a significant deficit of the upward-going events. Up/down ratio of these events was 0.56 ± 0.04(statistical) ± 0.01(systematic) and deviated from the expectation (unity) in the absence of neutrino oscillations by more than 8 standard deviations. The data can be naturally explained by neutrino oscillations, since the neutrinos change their flavor by neutrino oscillations after traveling long distances. This conclusion was supported by the muon/electron ratio of the neutrino events and by detailed studies of the upward-going muon events. We conclude that the atmospheric neutrino data from Super-Kamiokande give evidence for neutrino oscillations.
The atmospheric neutrino data are explained by the neutrino oscillations between muon-neutrinos and tau-neutrinos. However, it is possible to explain the data by introducing neutrino oscillations between muon-neutrinos and non-interacting sterile-neutrinos. We have carefully studied this possibility and found that the oscillations to sterile neutrinos are disfavored at about 99% C.L.
In summary, we conclude that neutrino oscillations are between muon-neutrinos and tau-neutrinos. The difference of the neutrino mass squared is between 2 〜 5 × 10ィイD13ィエD1eVィイD12ィエD1, and the mixing angle between the two neutrinos is sin22 θ > 0.88, at 90% C.L.
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