Search for new phenomena in the FCNC rare decays of neutral K meson

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FCNC rare decays, which are suppressed in the Standard model (SM), are sensitive to ontributions from unknown particles, and are ideal places to look for new phenomena beyond the SM and examine the various phenomenological models. We have analyzedthe K_L→π^0π^0X(X=μμ)process and made a search for X(214) particle in theμμfinal state. We settled the upper limit of the production and decay (1.6x10^-6). Also we have made a study of KL π^0π^0・υυdecay, which gives the ? parameter in the Kobayashi-Maskawa matrix, and gotten the results which made an order of magnitude improvement on the upper limit of the decay branching fraction.