Study of low energy ^^-KN interaction

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We have been studying the strong interaction between nucleon and K meson (kaon) by replacing an electron in the hydrogen atom with kaon, which has a strange quark. When, kaon stops in the gaseous hydrogen target, kaonic hydrogen atom is generated. The energy of the emitted x ray from the successive deexcitation gives energy shift and line width broadening from the electromagnetic value, especially for the transition to the ground. Therefore, the precise measurement of this X rays gives the most basic information of the ^^-KN interaction at the low energy limit. We have succeeded to observe the kaonic hydrogen 2p to 1s transition at KEK PS-E228 (KpX), and began the DAΦNE-DEAR experiment to deduce the isospin dependence of the interaction by detailed study both on K-d and K-p atom X rays observations.However, the way of the research is not necessarily smooth. The biggest problem is that the accelerator operation had been delayed substantially from the schedule away beyond the expectation. Moreover, we had been suffered by the huge background, which exceeded the expectation by 100 times more. On the other hand, this difficulty promoted a joint research with the theory with Akaishi et al. It is shown that the kaon may form a stable state inside the nuclei, based on the results i) our GSI experiment S218 revealed that the pi meson can exist inside the nuclei, and ii) our KpX result indicated strongly attractive nature between kaon and proton. If there exist deeply bound kaonic state inside nuclei, the purpose of this research program can be achieved more directly in more efficient way. By the present grant, we were able to start the search for the deeply-bound kaonic state at KEK (PS-E471).