Study of Solar Neutrinos at Super Kamiokande

Yusuke Koshio
University of Tokyo

July 1998

A thesis for the degree of PhD

Super-Kamiokande started taking data on the 1st of April in 1996. The data used for the analysis of solar neutrinos in this paper was obtained from the 31st of May, 1996 to the 23rd of June, 1997 and the total live time is 297.4 days. The energy threshold is 6.5MeV and the fiducial volume is 22.5 ktons. The observed $^8$B solar neutrino flux in Super-Kamiokande is \[ 2.42 \quad ^{+0.06}_{-0.06} (stat.) \quad ^{+0.13}_{-0.09} (sys.) \quad [\times 10^6/cm^2/sec]. \] The observed flux is smaller than the predicted by Bahcall and Pinsonneault (BP95). The ratio is \[ \frac{Data}{SSM(BP95)} = 0.365 \quad ^{+0.010}_{-0.010} (stat.) \quad ^{+0.019}_{-0.013} (sys.). \] Comparing this deficit with the result of the Homestake experiment, the solar neutrino problem is difficult to explain by changing solar models but is more naturally explained by neutrino oscillations. The solar neutrino flux of daytime and night-time agrees within the experimental errors and the result restricts the allowed neutrino oscillation parameters of the MSW effect. If we assume the large angle solutions to the solar neutrino problem, according to the MSW effect, neutrinos would regenerate through the earth. This analysis, which is independent of the solar models, excludes half of the large angle region which was obtained from the model-dependent analysis of the combined results from the past experiments. Within five years, the data sample from Super-Kamiokande should be large enough to study the whole large angle region. Seasonal flux differences were not seen but currently, the statistical relevance of the data sample is insufficient. No energy distortion was found within the errors. For the analysis of the energy spectrum, more statistics and smaller systematic errors are needed.


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KOSHIO, Yusuke (ICRR, Kamioka)

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