Keiichi Masutani

Kaonic Nuclei and Kaon-Nucleus Interactions

Although kaonic atoms provide valuable information concerning the K � -nucleus interaction at low energies, they cannot fully determine the K � -nucleus optical potential. We

The 4He(π−, n)3H reaction in a Δ-hole model

Abstract The 4 He (π − , n ) 3 H reaction in the resonance region is studied within the framework of the Δ-hole model. The basic mechanism of the pion absorption is assumed to be the ΔN → NN transition and the interaction parameters for this transition are determined through the analysis of the d(π+, p)p reaction. The present model is found to describe the main features of the observed cross sections for the 4 He (π − , n ) 3 H reaction, although it fails to explain the details of the angular distribution.

Negative-parity hyperons in the constituent quark model with meson-quark couplings

Abstract Negative-parity hyperons are systematically studied by means of the constituent quark model which incorporates the effect of the meson-quark coupling. Some unresolved puzzles of these resonances are investigated; the mass splitting between Λ(1405) and Λ(1520) can be realized by the difference of their self-energies due to the mesonic effect, without losing the general success of the constituent quark model; the coupling of the so-called “missing resonances” to the meson-baryon states are shown to be suppressed owing to the spin-flavor symmetry of the quark wave functions of the resonances. Although only the resonance contribution is considered for the meson-baryon scattering, the calculated amplitudes have the energy dependence consistent with the phase-shift analyses of the data. To obtain more quantitative agreement, however, the improvement of the model will be necessary; for example, non-resonant contribution is required to explain the isovector K N amplitude at low energies.

Nucleon Resonances in a Constituent Quark Model with Chiral Symmetry.

The mass spectra of nucleon resonances with spin 1/2, 3/2, and 5/2 are systematically studied in a constituent quark model with meson-quark coupling, which is inspired by the spontaneous breaking of chiral symmetry of QeD. Meson-quark coupling gives rise not only to the one-meson-exchange potential between quarks but also to the self-energy of baryon resonances, owing to the existence of meson-baryon decay channels. These two contributions are consistently taken into account in the present model. The gross properties of nucleon resonance spectra are reproduced fairly well, although the predicted mass of N(1440) is too large.

Absorptive contents of pionic-atom optical potentials and the anomaly problem

Abstract We show that the imaginary parts of a pionic-atom optical potential cannot be a candidate, by themselves, for the explanation of the anomaly problem, when two-nucleon absorption is assumed to be described effectively by quadratic forms of nuclear densities. By examining multinucleon absorption processes, we find a criterion that will help to determine whether a potential fit to pionic-atom data (especially to anomalous data), is acceptable.

Investigation of the 3d pionic atom anomaly in low-energy pion scattering

Abstract We find it possible to investigate the anomalously small shifts and widths observed in the 3d state of heavy pionic atoms by means of π (particularly π−)-nucleus scattering below 30 MeV. To achieve this goal, however, reasonably accurate data must become available and be analyzed theoretically with care.

Baryon Resonances in a Constituent Quark Model

The role of pseudoscalar mesons in the dynamical properties of baryon resonances is examinedby means of the constituent quark mod el with meson-quark coupling. Since resonance properties are closely relatedto meson-baryon scatterings, it is important to includ e reaction processes as well as mass spectra in the analysis. The isobar model is employed to properly extract information of baryon resonances from reaction amplitudes. Prominent features of resonance spectra are reproduced fairly well by taking account of baryon selfenergies due to meson-quark coupling. Several problems remained in the static quark model of baryons, e.g. Λ(1405)-Λ(1520) mass difference, are also studied. Brief comments on some improvements of the present model are given.

Effects of baryon resonances on nucleon–nucleon interactions in a quark model

Abstract Nucleon–nucleon S -wave scattering is studied by using the resonating group method, which includes not only the nucleon and Δ but also their excited states. As quark–quark interactions, the one-gluon-exchange and one-pion-exchange potentials are employed with phenomenological confinement potential. Compared with the N and Δ , coupling effects induced by excited-state baryons are small.

Radiative decays of kaonic hydrogen and a constituent quark model with meson-quark couplings

Abstract The low-energy K N interaction is satisfactorily described by the constituent quark model with meson-quark couplings when the model is supplemented with a separable potential representing “non-isobar” contributions. The model is applied to the study of the radiative decays of kaonic hydrogen. The electromagnetic couplings of the quark- and mesonic-currents are defined in a phenomenological way. The calculated radiative decay widths of kaonic hydrogen agree with the experimental values fairly well. The meson-baryon interaction plays an important role as the initial state interaction in these radiative transitions.