Abstract:
A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the $\pi N \rightarrow \eta N$ and $\eta N \rightarrow \eta N$ amplitudes using as input the latest data for the dominant $S_{11}$ $\pi N$ elastic scattering partial wave following suggestions of Prof. G. H\"{o}hler. The sign error in the numerical evaluation of the dispersion integral in the original publication is eliminated. The remaining weighted data set for the $\pi N \rightarrow \eta N$ total and differential cross sections is used as in the original publication. The correction of the numerical error influences the $\eta N$ cusp effect and improves the quality of the fit to the input data. However, our new result for the $\eta N$ scattering length, $a_{\eta N }= (0.717 \pm 0.030) + i (0.263 \pm 0.025)$ fm, is a sole consequence of the correction of the $S_{11}$ input and suggests that the $\eta d$ system is unbound or loosely bound.

Abstract:
A partial-wave analysis of 9082 eta pi+ pi- n events produced in the reaction pi- p --> eta pi+ pi- n at 18.3 GeV/c has been carried out using data from experiment 852 at Brookhaven National Laboratory. The data are dominated by J^{PC} = 0^{-+} partial waves consistent with observation of the eta(1295) and the eta(1440). The mass and width of the eta(1295) were determined to be 1282 +- 5 MeV and 66 +- 13 Mev respectively while the eta(1440) was observed with a mass of 1404 +- 6 MeV and width of 80 +- 21 MeV. Other partial waves of importance include the 1++ and the 1+- waves. Results of the partial wave analysis are combined with results of other experiments to estimate f1(1285) branching fractions. These values are considerably different from current values determined without the aid of amplitude analyses.

Abstract:
A combined analysis of photoproduction data on \gamma p to \pi N, eta N was performed including the data on K Lambda and K Sigma. The data are interpreted in an isobar model with s--channel baryon resonances and pi, rho,(omega), K, and K^* exchange in the t--channel. Three baryon resonances have a substantial coupling to eta N, the well known N(1535)S_{11}, N(1720)P_{13}, and N(2070)D_{15}. The inclusion of data with open strangeness reveals the presence of further new resonances, N(1840)P_{11}, N(1875)D_{13} and N(2170)D_{13}.

Abstract:
Within a chiral constituent quark model approach, $\eta$-meson production on the proton {\it via} electromagnetic and hadron probes are studied. With few parameters, differential cross-section and polarized beam asymmetry for \gamma p \to \eta p$ and differential cross section for $\pi^- p\to \eta n$ processes are calculated and successfully compared with the data in the center-of-mass energy range from threshold up to 2 GeV. The five known resonances $S_{11}(1535)$, $S_{11}(1650)$, $P_{13}(1720)$, $D_{13}(1520)$, and $F_{15}(1680)$ are found to be dominant in the reaction mechanisms in both channels. Possible roles plaied by new resonances are also investigated and in the photoproduction channel, significant contribution from $S_{11}$- and $D_{15}$-resonances, with masses around 1715 MeV and 2090 MeV, respectively, are deduced. For the so-called missing resonances, no evidence is found within the investigated reactions. The helicity amplitudes and decay widths of $N^* \to \pi N, \eta N$ are also presented, and found consistent with the PDG values.

Abstract:
The coupled pi+N, eta+N, gamma+N systems are described by a K-matrix method. The parameters in this model are adjusted to get an optimal fit to pi+N -> pi+N, pi+N -> eta+N, gamma+N -> pi+N and gamma+N -> eta+N data in an energy range of about 100 MeV each side of the eta-threshold. The coupling of photons to the N(1535) state is extracted and also an alternative to the current S11(gamma+N -> pi+N) amplitudes suggested. Expansions are given for the eta-eta and gamma-eta amplitudes in terms of the eta-momentum. Effects of interference of this state with background potential interactions are discussed and experimental consequences are indicated.

Abstract:
The measurements of reactions $\pi^- p \to \pi^- \pi^+ n$ and $\pi^+ n \to \pi^+ \pi^- p$ on polarized targets at CERN found a strong dependence of pion production amplitudes on nucleon spin. Analyses of recent measurements of $\pi^- p \to \pi^0\pi^0 n$ reaction on unpolarized targets by GAMS Collaboration at 38 GeV/c and BNL E852 Collaboration at 18 GeV/c use the assumption that pion production amplitudes do not depend on nucleon spin, in conflict with the CERN results on polarized targets. We show that measurements of $\pi^- p \to \pi^0\pi^0 n$ and $\pi^- p \to \eta\eta n$ on unpolarized targets can be analysed in a model independent way in terms of 4 partial-wave intensities and 3 independent interference phases in the mass region where $S$- and $D$-wave dominate. We also describe model-independent amplitude analysis of $\pi^- p \to \pi^0\pi^0 n$ reaction measured on polarized target, both in the absence and in the presence of $G$-wave amplitudes. We suggest that high statistics measurements of reactions $\pi^- p \to \pi^0 \pi^0 n$ and $\pi^- p \to \eta\eta n$ be made on polarized targets at Protvino IHEP and at BNL, and that model-independent amplitude analyses of this polarized data be performed to advance hadron spectroscopy on the level of spin dependent production amplitudes.

Abstract:
The production of eta' mesons in photon- and hadron-induced reactions has been revisited in view of the recent additions of high-precision data to the world data base. Based on an effective Lagrangian approach, we have performed a combined analysis of the free and quasi-free gamma N -> eta' N, N N -> N N eta', and pi N -> eta' N reactions. Considering spin-1/2 and -3/2 resonances, we found that a set of above-threshold resonances {S_{11}, P_{11}, P_{13}}, with fitted mass values of about M_R=1925, 2130, and 2050 MeV, respectively, and the four-star sub-threshold P_{13}(1720) resonance reproduce best all existing data for the eta' production processes in the resonance-energy region considered in this work. All three above-threshold resonances found in the present analysis are essential and indispensable for the good quality of the present fits.

Abstract:
Angular distributions in the final state of pi0-eta photoproduction on nucleons are considered. As a formal base the familiar isobar model is used in which the (pi0 eta N) state is a product of the resonance decay into eta-Delta(1232) and pi-S_{11}(1535) channels. One of the principal assumptions used is that in the actual energy region the reaction is dominated by a single resonance state. The developed formalism can serve as a tool for testing spin and parity of that resonance.

Abstract:
We calculate formation spectra of eta-nucleus systems in (pi,N) reactions with nuclear targets, which can be performed at existing and/or forthcoming facilities, including J-PARC, in order to investigate eta-nucleus interactions. Based on the N^*(1535) dominance in the eta N system, eta-mesic nuclei are suitable systems for study of in-medium properties of the N^*(1535) baryon resonance, such as reduction of the mass difference of N and N^* in nuclear medium, which affects level structure of the eta and N^*-hole modes. We find that clear information on the in-medium N^*- and eta-nucleus interactions can be obtained through the formation spectra of the eta-mesic nuclei. We also discuss the experimental feasibilities by showing several spectra of (pi,N) reactions calculated with possible experimental settings. Coincident measurements of pi N pairs from the N^* decays in nuclei help us to reduce backgrounds.

Abstract:
We calculate formation spectra of eta-nucleus systems in (pi,N) reactions with nuclear targets, which can be performed at existing and/or forthcoming facilities, including J-PARC, in order to investigate eta-nucleus interactions. Based on the N^*(1535) dominance in the eta N system, eta-mesic nuclei are suitable systems for study of in-medium properties of the N^*(1535) baryon resonance, such as reduction of the mass difference of N and N^* in nuclear medium, which affects level structure of the eta and N^*-hole modes. We find that clear information on the in-medium N^*- and eta-nucleus interactions can be obtained through the formation spectra of the eta-mesic nuclei. We also discuss the experimental feasibilities by showing several spectra of (pi,N) reactions calculated with possible experimental settings. Coincident measurements of pi N pairs from the N^* decays in nuclei help us to reduce backgrounds.