Study of Neutrino Mass and Mixing by Long Baseline Neutrino Oscillations : from K2K to a Next-Generation Experiment

本研究项目的目的之一是进行K2K实验。K2K实验是第一个加速器长基线中微子振荡实验。该实验的目的是通过人工产生的介子中微子束来证实超级神冈探测器对大气中微子振荡的发现。在超级神冈总共观测到112个中微子事件；如果ν_μ不振荡，我们预计会发生158.1（+9.2-8.6）次事件。ν_μ振荡引起的中微子能谱畸变也被观察到。即使没有中微子振荡，我们观测到这些结果的概率也只有0.0015%。相应的显著性为4.3σ。对ν_μ消失的双味中微子振荡进行了分析。得到的最佳拟合振荡参数为（Δm^2,sin^22θ）=（2.8×10^<-3> eV^2,1.0）。这一结果与超级神冈探测器的大气中微子振荡分析相一致。因此，成功地完成了K2K实验的目的。本研究项目的另一个重要目的是研发T2K中微子束流线，特别是研制获得高强度中微子束的最重要的束流线元素——三态磁角。T2K是日本下一个从J-PARC到超级神冈的长基线中微子振荡实验。研发的主要目标是第一个喇叭，它应该在非常严重的辐射和热负荷下工作。我们对电冲击和热冲击进行了广泛的分析，通过这些研究，我们优化了喇叭的结构和尺寸。为了验证这种设计的喇叭在高强度光束冲击下的耐久性，已经建造了一个原型。初步测试显示了预期的性能。

One of the purposes of the present research project is to conduct the K2K experiment. The K2K experiment is the first accelerator long baseline neutrino oscillation experiment. The purpose of the experiment is to confirm the Super-Kamiokande's discovery of atmospheric neutrino oscillation by using artificially produced muon neutrino beams. In total,112 neutrino events are observed in Super-Kamiokande ; we expect 158.1(+9.2-8.6) events if ν_μ does not oscillate. The neutrino energy spectrum distortion caused by ν_μ oscillation is also seen. The probability that we would observe these results due to statistical fluctuation even if there is no neutrino oscillation is 0.0015%. The corresponding significance is 4.3σ. A two-flavor neutrino oscillation analysis, with ν_μ disappearance, is performed. The best-fit oscillation parameters obtained are (Δm^2,sin^22θ)=(2.8×10^<-3> eV^2,1.0) in the physical region. This results is consistent with the Super-Kamiokande's atmospheric neutrino oscillation analysis. Therefore the purpose of the K2K experiment has been successfully accomplished.Another important purpose of the present research project is R & D of the T2K neutrino beam line, in particular, that of triplet magnetic horns which are the most important beam line elements to obtain high intensity neutrino beams. T2K is the next Japanese accelerator long baseline neutrino oscillation experiment from J-PARC to Super-Kamiokande. The main objective of R & D is the first horn which should operate under very severe radiation and thermal loads. We have made extensive analyses of electrical and thermal shocks, and through these studies we have optimized the structure and size of the horn. To verify the endurance of thus designed horn against the shocks due to high intensity beams, a prototype has been constructed. Initial testing has shown expected performances.