Abstract
Research on kaonic nuclear bound states is a completely new field. This nuclear system consists of an anti-K-meson (a meson having one strange quark), called a kaon, bound in a nucleus. The kaon behaves as a quasi-on-mass-shell particle and forms its own quantum state in the nuclear system. Mesons are known to behave as virtual particles in nuclei, repeatedly being created and annihilated rapidly so as to form a nuclear potential. In contrast, R.H. Dalitz suggested in the 1960s that the Λ(1405) can be a peculiar “\({\bar K}\!N\)” system, i.e., a molecule-like hadronic cluster composed of a nucleon and a \({\bar K}\) meson. However, there was no clear experimental evidence to strongly support this hypothesis until recently. In 1997, atomic kaonic hydrogen data suggested a strong attraction between kaon and proton, which is sufficient to form a nuclear bound state. This data triggered a variety of intensive search experiments to confirm the discovery. This is because the existence of a kaonic nuclear state is a natural interpretation of the Λ(1405), to be a molecule-like “\({\bar K}\!N\)” hadronic cluster. Very recently the first clear evidence for a “K −pp” bound system (composed of a K − and two protons in the charge base notation) formation was obtained. Even more interesting, a simple form(structure)-factor analysis of the experimental data on the “K −pp” signal suggested that this state may be extremely compact compared to the normal internucleon distance in nuclei, which may lead to exploring physics beyond the normal nuclear density in a quantum equilibrium. This article will discuss why and how kaonic nuclear bound states were pursued, how they were finally observed, and where such experimental research may lead us in the future.
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Acknowledgements
I appreciate intensive discussions that were deeply related to this paper with Dr. Takumi Yamaga. I would like to thank Dr. Tadashi Hashimoto for various suggestions and also Dr. Rie Murayama, Dr. Kenta Itahashi, and Dr. Fuminori Sakuma. I’m grateful to Dr. Benjamin F. Gibson, Dr. Hideyuki Sakai, Professor Emiko Hiyama, Professor Tetsuo Hyodo, and Professor Hiroyuki Noumi for valuable comments. Finally, I would like to thank Ms. Harumi Iwasaki for continuous support in various aspects of this work.
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Iwasaki, M. (2023). Kaonic Nuclei from the Experimental Viewpoint. In: Tanihata, I., Toki, H., Kajino, T. (eds) Handbook of Nuclear Physics . Springer, Singapore. https://doi.org/10.1007/978-981-19-6345-2_37
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