Research for neutrino masses using solar neutrinos

中微子质量的研究是利用超级神冈探测器1258天的太阳中微子数据进行的。超级神冈能够以前所未有的高精度测量太阳中微子通量的昼夜变化和太阳中微子的能谱。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，如果在这些测量中发现异常，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，则应该强烈约束中微子振荡参数（中微子质量差和混合角）。本研究通过改进分析方法，使我们能够分析低至5 MeV的数据。对于日/夜分析，太阳中微子通量的日/夜不对称性为3.3%±2.2% (stat.)+1.3/-1.2% （sys.）。虽然它离零有1.3 sigma的距离，但在统计上并不显著。对于太阳中微子的光谱形状分析，观测到的能谱形状与预期的光谱形状一致，置信水平（CL）为39%。从这些测量中得到了对中微子振荡参数的强约束，太阳中微子问题的小混合角解在95% CL左右是不利的。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亏缺，大的混合溶液在太阳中微子振荡中是有利的。

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.