The ''K^-pp'' system investigated with a coupled-channel complex scaling method

Nuclear systems with anti-kaons (kaonic nuclei) are expected to have exotic nature such as formation of a dense state, due to the strong attraction between an anti-kaon and a nucleon. In this article, the current theoretical studies of the most essential kaonic nucleus ''K^-pp'' are reviewed and our study of ''K^-pp'' based on a coupled-channel Complex Scaling Method is reported. With a new version of our chiral SU(3)-based potential constrained by a precise measurement of kaonic hydrogen atom (SIDDHARTA experiment), the ''K^-pp'' state is found to be rather shallowly bound: B_K^-pp = 15 ∼ 22 MeV and Γ_M = 20 ∼ 50 MeV. The ''K^-pp'' seems to have another pole, and it might possibly have so-called a double-pole structure, similarly to the Λ(1405). Recent results of the K^-pp search experiments (J-PARC E15 and E27) are discussed in comparison with several new theoretical studies. (author)