In-medium modification of the eta mass is discussed in the context of partial restoration of chiral symmetry in nuclear medium. We emphasize that the U_A(1) anomaly effects causes the eta-eta mass difference necessarily through the chiral symmetry br
eaking. As a consequence, the eta mass is expected to be reduced by order of 100 MeV in nuclear matter where about 30% reduction of chiral symmetry takes place. The strong attraction relating to the eta mass generation eventually implies that there should be also a strong attractive interaction in the scalar channel of the eta-N two-body system. We find that the attraction can be strong enough to form a bound state.
Hadronic composite states are introduced as few-body systems in hadron physics. The $Lambda(1405)$ resonance is a good example of the hadronic few-body systems. It has turned out that $Lambda(1405)$ can be described by hadronic dynamics in a modern t
echnology which incorporates coupled channel unitarity framework and chiral dynamics. The idea of the hadronic $bar KN$ composite state of $Lambda(1405)$ is extended to kaonic few-body states. It is concluded that, due to the fact that $K$ and $N$ have similar interaction nature in s-wave $bar K$ couplings, there are few-body quasibound states with kaons systematically just below the break-up thresholds, like $bar KNN$, $bar KKN$ and $bar KKK$, as well as $Lambda(1405)$ as a $bar KN$ quasibound state and $f_{0}(980)$ and $a_{0}(980)$ as $bar KK$.
We shed light upon the eta mass in nuclear matter in the context of partial restoration of chiral symmetry, pointing out that the U_{A}(1) anomaly effects causes the eta-eta mass difference necessarily through the chiral symmetry breaking. As a conse
quence, it is expected that the eta mass is reduced by order of 100 MeV in nuclear matter where partial restoration of chiral symmetry takes place. The discussion given here is based on Ref. [1].
The appearance of some papers dealing with the $K^- d to pi Sigma n$ reaction, with some discrepancies in the results and a proposal to measure the reaction at forward $n$ angles at J-PARC justifies to retake the theoretical study with high precision
to make accurate predictions for the experiment and extract from there the relevant physical information. We do this in the present paper showing results using the Watson approach and the truncated Faddeev approach. We argue that the Watson approach is more suitable to study the reaction because it takes into account the potential energy of the nucleons forming the deuteron, which is neglected in the truncated Faddeev approach. Predictions for the experiment are done as well as spectra with the integrated neutron angle.
The internal structure of the resonant Lambda(1405) state is investigated based on meson-baryon coupled-channels chiral dynamics. We evaluate Lambda(1405) form factors which are extracted from current-coupled scattering amplitudes in meson-baryon deg
rees of freedom. Using several probe currents and channel decomposition, we find that the resonant Lambda(1405) state is dominantly composed of widely spread Kbar around N, with escaping pi Sigma component.
The scattering length and effective range of the piSigma channel are studied in order to characterize the strangeness S=-1 meson-baryon scattering and the Lambda(1405) resonance. We examine various off-shell dependence of the amplitude in dynamical c
hiral models to evaluate the threshold quantities with the constraint at the KbarN threshold. We find that the piSigma threshold parameters are important to the structure of the Lambda(1405) resonance and provide further constraints on the subthreshold extrapolation of the KbarN interaction.
We study the origin of the resonances associated with pole singularities of the scattering amplitude in the chiral unitary approach. We propose a natural renormalization scheme using the low-energy interaction and the general principle of the scatter
ing theory. We develop a method to distinguish dynamically generated resonances from genuine quark states [Castillejo-Dalitz-Dyson (CDD) poles] using the natural renormalization scheme and phenomenological fitting. Analyzing physical meson-baryon scatterings, we find that the Lambda(1405) resonance is largely dominated by the meson-baryon molecule component. In contrast, the N(1535) resonance requires a sizable CDD pole contribution, while the effect of the meson-baryon dynamics is also important.
We study the behavior with the number of colors (Nc) of the two poles associated to the Lambda(1405) resonance obtained dynamically within the chiral unitary approach. The leading order chiral meson-baryon interaction manifests a nontrivial Nc depend
ence for SU(3) baryons, which gives a finite attractive interaction in some channels in the large Nc limit. As a consequence, the SU(3) singlet (Kbar N) component of the Lambda(1405) survives in the large Nc limit as a bound state, while the other components dissolve into the continuum. The Nc dependence of the decay widths shows different behavior from the general counting rule for a qqq state, indicating the dynamical origin of the two poles for the Lambda(1405) resonance.
The structure of N(1535) is discussed in dynamical and symmetry aspects based on chiral symmetry. We find that the N(1535) in chiral unitary model has implicitly some components other than meson-baryon one. We also discuss the N(1535) in the chiral doublet picture.
We explore a possibility to generate exotic hadrons dynamically in the scattering of hadrons. The s-wave scattering amplitude of an arbitrary hadron with the Nambu-Goldstone boson is constructed so as to satisfy the unitarity condition and the chiral
low energy theorem. We find that the chiral interaction for the exotic channels is in most cases repulsive, and that the strength of the possible attractive interaction is uniquely determined. We show that the attractive interaction in exotic channels is not strong enough to generate a bound state, while the interaction in nonexotic channel generate bound states which are considered to be the origin of some resonances observed in nature.
