Study of Baryon Excitations in Nuclei by Electromagnetic Interactions

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1) The nuclear effective interactions above the mesonproduction : We constructed a new formalism which enables us to construct the effective interaction of baryons andmesons in nuclei which seems a natural extension of the traditional nuclear interactions. The idea is to remove any virtual interactions at all the real processes by the unitary transformation. As a result, we found that our one-poin-exchange and static two-pion exchange potentials coincide with the conventional potentials. At the same time, we obtained general baryon-baryon and meson-baryon potentials, and the exchange electromagnetic currents. The exchange current which does not include isobar excitation is the same as the conventional one, and the current associated with isobar excitation is consistent with the baryon-baryon force. This formalism enables us to study the strong and electromagnetic phenomena in nuclei including the meson production and annihilation processes. The basic interaction for pion-nucleon scatte … More ring and pion photoproduction was successfully studied. Also threshold neutral pion production by p-p collision was studied. At present, study of the reactions (pi, eta), (gamma, eta) is in progress.2) Pion-nuclear reaction : As a first step toward the study of nuclear response by electromagentic interaction above mesonproduction threshold, we studied the properties of isobar spreading potential through the elastic and charge exchange pion-nucleus scattering and the analysing power around the (3,3) resonance. For this purpose, we gave a formalism of isobar-doorway state model which enables us to study the pion scattering by the open shell nuclei. By investigating pion scattering by ^<12>C,^<13>C,^<15>N and ^<16>O,we clarified existence of the Lane term as well as the spin-orbit term which are energy-independent. The enphasis should be placed on that these two terms allow us to describe correctly the absolute magnitude of charge-exchange scattering and also signs of analysing power. At present, pion-photoproduction on the p-shell nuclei around (3,3) resonance is under investigation based on the present model.3) Baryon Structure in chiral soliton quark model : Structure of vertex functions in the effective hadorn interaction is essentially important in the nuclear phenomena at high momentum transfer. As a first step for thsi, we studied magnetic moments of baryons by adopting the SU (3) chiral soliton quark model inspired by the QCD theory. By including quark-mass difference between u (and/or d) and s as a symmetry breaking term, we obtained a good agreement between the theory and the experiment for the Octet baryons, and also results for the Decuplet baryons similar to the lattice calculation. These results imply large reliability of the model for the structure of vertex function. Also the Godfried sum was studied successfully. Less