Investigation of Neutrino Oscillation Phenomena toward Determination of Structure of Lepton Flavor Mixing

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1. We have proposed a concrete setting for determination of CP violating phase and neutrino mass hierarchy, called the Tokai to Kamioka-Korea (T2KK), in which an intense neutrino beam from J-PARC located in Tokai village is received by the identical two detectors, one placed in Kamioka and the other in Korea with each 0.5 megaton water. We have shown that T2KK can resolve the eight-fold parameter degeneracy and can determine all the unknown elements of MNS matrix to the accuracy comparable to those achieved by other project proposed in the world, e.g., the neutrino factory, as far as the mixing angle theta_13 exists within the region of sensitivity of the next generation long-baseline experiments such as T2K (Tokai-to-Kamioka). Toward the realization of this idea, the two international workshops were held in Seoul, Korea, in 2005 and 2006. T2KK is considered to be one of the good candidates for future project beyond T2K. We have also put forward the theoretical understanding of the pro … More perty of parameter degeneracy by uncovering the property called "decoupling between the degeneracies" which implies that each two pair of the degeneracies can be resolved independent of the presence of the other degeneracies.2. In 2003 wo have proposed idea of reactor experiments for measuring theta_13 by using the near-far identical two detector setting as a pure measurement of theta_13. We then suggested that the theta_23 degeneracy can be resolved by the reactor-accelerator combined method. Based on this idea and in collaboration with the two T2K experimentalists, we have examined in detail the sensitivity of resolving the theta_23 degeneracy that can be achieved by the method. The results indicates that the resolving power of the theta_23 degeneracy by this method is greater than any other methods proposed so far in region of relatively large theta_13, sin~2 2theta_13 > 0.5-0.6.3. The head investigator proposed (with Alexei Smirnov) the new concept called the "quark-lepton complementarity" (QLC) based on a hint given by experimental data that the Cabibbo angle and the solar angles add up to ji/4. It may imply a new approach to the problem of how quarks and leptons are unified. We have constructed some concrete scenarios on how QLC can be embedded into grand unified theories, and proposed how they can be tested experimentally. All of them involve a novel feature that there is the maximal angle in the (1-2) sector of the lepton flavor mixing. Less