Research for neutrino masses using solar neutrinos

中微子质量的研究是利用Super-Kamiokande卫星1258天的太阳中微子数据进行的。Super-Kamiokande能够以前所未有的高精度测量太阳中微子通量和太阳中微子能谱的昼夜变化。它们纯粹是由于中微子作为基本粒子的性质，而不依赖于太阳模型的细节。因此，如果发现这些测量中的异常，它应该是中微子质量的直接证据。相反，如果没有观测到异常，它应该强烈约束中微子振荡参数（中微子质量差和混合角）。通过改进分析方法，我们可以分析低至5 MeV的数据。对于昼夜分析，得到的太阳中微子通量的昼夜不对称性为3.3%±2.2%（stat.）+ 1.3/-1.2%（系统）。虽然它与零相差1.3 sigma，但在统计学上并不显著。对于太阳中微子能谱形状的分析，观测到的能谱形状与预期的能谱形状在39%的置信水平（CL）下是一致的。从这些测量结果中，我们得到了对中微子振荡参数的强约束，小混合角的太阳中微子问题的解是不利的，CL约为95%，观测到的太阳中微子通量仅为标准太阳模型预期通量的45%.考虑到观测到的绝对通量的不足，在太阳中微子振荡中，大的混合解是有利的。

Research for neutrino masses has been performed using the 1258 days' solar neutrino data taken by Super-Kamiokande. Super-Kamiokande is able to measure day/night variation of solar neutrino flux and energy spectrum of solar neutrinos with unprecedented high accuracy. They are purely due to property of neutrinos as an elementary particle and it does not depend upon the details of solar models. So, if anomaly in those measurements are found, it should be a direct evidence of neutrino masses. To the contrary, if anomaly is not observed, it should strongly constrain neutrino oscillation parameters (neutrino mass difference and mixing angle). By improving the analysis method by this research, we are able to analyze data down to 5 MeV.For the day/night analysis, the obtained day/night asymmetry of the solar neutrino flux was 3.3%±2.2% (stat.)+1.3/-1.2% (sys.). Although it is 1.3 sigma away from zero, it is not statistically significant. For the analysis of the spectrum shape of solar neutrinos, observed shape of the energy spectrum is consistent with the expected spectrum shape with 39% confidence level (CL). From those measurements, strong constraint on the neutrino oscillation parameters are obtained and small mixing angle solution of the solar neutrino problem is disfavored with about 95% CL.The observed flux of solar neutrinos is only 45% of the expected flux from the standard solar models. Taking into account the deficit of the observed absolute flux, large mixing solutions are favored in the solar neutrino oscaillations.