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内容記述 |
The K<SUP>0</SUP><SUB>L</SUB> → π<SUP>0</SUP>νν decay is a golden channel to study the origin of CP violation. The branching ratio is proportional to the parameter η, which measures the magnitude of the CP violation in the CKM matrix; the theoretical uncertainties are so small that the measurement allows us to make a precise test of the Standard Model of CP violation.<br /> Experiment E391a was designed to measure K<SUP>0</SUP><SUB>L</SUB> → π<SUP>0</SUP>νν decay with a goal of 3・10<SUP>-10</SUP> single-event sensitivity. A K<SUP>0</SUP><SUB>L</SUB> → π<SUP>0</SUP>νν decay is searched for by a signal of K<SUP>0</SUP><SUB>L</SUB> → π<SUP>0</SUP>(π<SUP>0</SUP> → γγ) + nothing. The energies and positions of the gammas were measured with a CsI calorimeter. The "nothing" was confirmed by no additional signals in the veto system, which covered the whole decay region. Several unique techniques were developed for E391a, such as a well-collimated. "pencil" beam, differential pumping, a double-decay chamber and highly sensitive veto detectors to cover almost the whole 4π geometry.<br /> During the beam time in February-June 2004, we collected about 6Tb of data, equivalent to a full 60 days of operation. As the first step, we performed an analysis of one day of data, which is a main topic of the present thesis. Good quality and stability of the data were confirmed by this analysis. All detectors were calibrated by using punch-through and/or cosmic muons. In addition, the CsI calorimeter was calibrated more precisely by using γ's from the π<SUP>0</SUP>'s produced with an aluminum target. The reconstruction of neutral decays (K<SUP>0</SUP><SUB>L</SUB> → 3π<SUP>0</SUP>, 2π<SUP>0</SUP>, γγ) showed good agreement with MC simulations and in the number of the K<SUP>0</SUP><SUB>L</SUB> yield. The pure signal and background samples of K<SUP>0</SUP><SUB>L</SUB> → 3π<SUP>0</SUP>, 2π<SUP>0</SUP>, γγ were used for a veto study, and showed good agreement of the acceptance loss due to the veto cuts.<br /> In the 2γ sample, which contains candidates for K<SUP>0</SUP><SUB>L</SUB> → π<SUP>0</SUP>νν decays, we observed various sources of background events surrounding the signal box. They were from the other K<SUP>0</SUP><SUB>L</SUB> decays and from halo neutron interactions; in addition, we found a clear effect due to the interactions of the beam-core neutrons with the membrane separating the two vacuum chambers, which fell down into the beam axis, accidentally.<br /> Finally, we opened the signal box and observed no event inside. The single-event sensitivity for K<SUP>0</SUP><SUB>L</SUB> → π<SUP>0</SUP>νν decay was estimated to be 8.3 × 10<SUP>-7</SUP>. Thus led to an upper limit of the branching ratio of 1.9・ 10<SUP>-6</SUP> at the 90% confidence level.<br /> We conclude that the analysis of the one-day data was very valuable to understand the performance of the experiment and to develop new software techniques for the analysis. The. results of this study were reflected in an upgrade of the experimental setup for the next run, RUN II, which started in the middle of January, 2005. |
要旨・審査要旨 (213.9 kB)
