Physics of Lepton Flavor Mixing

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During the research term 1997-98 I concentrated on two topics, (1) CP Violation in the Lepton Sector, and (2) Dark Matter Neutrino Hypothesis ; experimental constraints and how to test it.(1) I, in collaboration with Hiroshi Nunokawa (Univ.de Campinas, Brazil), studied the problem of how to measure leptonic CP violation in long-baseline neutrino oscillation experiments. There exists a notorious problem in such measurement ; it is the problem of the matter effect pollusion. That is, the genuine CP violation effect due to the leptonic Kobayashi-Maskawa phase could be masked by that due to matter effect in the IEarth. We have presented a formalism based on matter effect peturbation theory by which one can obtain bird-eye view of the interplay between these two effects. It will enable us to select out the most feasible channel of the experiment searching for the genuine phase CP violation. Moreover, we have proposed a new strategy for sensitive detection of CP violation by setting up an intermediate detector in a dedicated long-baseline experimennt.(2) The neutrino dark matter hypothesis has been topics of interests as an interesting possibility of explaining structure formation in a consistent way with COBE data. I, in collaboration with Osamu Yasuda (TMU), demonstrated that there exist severe constraints on that hypothesis in the standard three-flavor mixing scheme due to the solar and the atmospheric neutrino observation as well as the reactor and accelerator data. We have shown with constraints imposed by CHOOZ reactor experiment that the dark matter neutrinos must have an almost degenerate mass spectrum. Furthermore, we have shown that if neutrinos are Majorana particles this possibility will be severely tested within 5 years by the coming new double beta decay experiment.