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The full complex phase structure of the meson-baryon reaction amplitude in coupled channels approach is investigated, including also the photon-baryon channel. The result may be viewed as a generalization of the well-known Watson's theorem. Furthermore, the complex phase structure is exhibited for the pole and nonpole parts of the reaction amplitude in such a way that it will serve as a convenient common starting point for constructing models with different levels of approximation, in particular, for building isobar models where the basic properties of the $S$-matrix can be maintained. Such models should be useful, especially, in coupled multichannel calculations, where a large amount of experimental data are considered in resonance analyses, a situation encountered in modern baryon spectroscopy. In particular, it is shown that the unitarity of the pole part of the $T$-matrix arises automatically from the dressing mechanism inherent in the basic scattering equation. This implies that no separate conditions are required for making this part of the resonance amplitude unitary as it has been done in some of the existing isobar models.

By exploiting the underlying symmetries of the relative phases of the pseudoscalar meson photoproduction amplitude, we provide a consistent and explicit mathematical derivation of the completeness condition for the observables in this reaction. In particular, we determine all the possible sets of four double-spin observables that resolve the phase ambiguity of the amplitude in transversity basis up to an overall phase. The present work substantiates and corroborates the original findings of Chiang and Tabakin [Phys. Rev. C 55, 2054 (1997)]. It is found, however, that the completeness condition of four double-spin observables to resolve the phase ambiguity holds only when the relative phases do not meet the condition of equal magnitudes. In situations where this condition occurs, it is shown that one needs extra chosen observables, resulting in the minimum number of observables required to resolve the phase ambiguity reaching up to eight, depending on the particular set of four double-spin observables considered. Furthermore, a way of gauging when the condition of equal magnitudes occurs is provided.

Spin-observables in pseudoscalar meson photoproduction is discussed. This work is complementary to the earlier works on this topic. Here, the reaction amplitude is expressed in Pauli-spin basis which allows to calculate all the observables straightforwardly. We make use of the fact that the underlying reflection symmetry about the reaction plane can be most conveniently and easily exploited in this basis to help finding the non-vanishing and independent observables in this reaction. The important issue of complete experiments is reviewed. By expressing the reaction amplitude in Pauli-spin basis, many sets of eight observables - distinct from those found in earlier works from the amplitude in transversity basis - capable of determining the reaction amplitude up to an overall phase are found. It is also shown that some of the combinations of the spin observables are suited for studying certain aspects of the reaction dynamics. We, then, carry out a (strictly) model-independent partial-wave analysis, in particular, of the peculiar angular behavior of the beam asymmetry observed in eta' photoproduction very close to threshold [P. Levi Sandri et al. 2015 Eur. Phys. J. A 51, 77]. This work should be useful, especially, for newcomers in the field of baryon spectroscopy, where the photoproduction reactions are a major tool for probing the baryon spectra., 47 pages, 3 figures, 6 tables

A theory of two-pion photo- and electroproduction off the nucleon is derived considering all explicit three-body mechanisms of the interacting $\pi\pi N$ system. The full three-body dynamics of the interacting $\pi\pi N$ system is accounted for by the Faddeev-type ordering structure of the Alt-Grassberger-Sandhas equations. The formulation is valid for hadronic two-point and three-point functions dressed by arbitrary internal mechanisms provided all associated electromagnetic currents are constructed to satisfy their respective (generalized) Ward-Takahashi identities. It is shown that coupling the photon to the Faddeev structure of the underlying hadronic two-pion production mechanisms results in a natural expansion of the full two-pion photoproduction current $M_{\pi\pi}^\mu$ in terms of multiple dressed loops involving two-body subsystem scattering amplitudes of the $\pi\pi N$ system that preserves gauge invariance as a matter of course order by order in the number of (dressed) loops. A closed-form expression is presented for the entire gauge-invariant current $M_{\pi\pi}^\mu$ with complete three-body dynamics. Individually gauge-invariant truncations of the full dynamics most relevant for practical applications at the no-loop, one-loop, and two-loop levels are discussed in detail. An approximation scheme to the full two-pion amplitude for calculational purposes is also presented. It approximates, systematically, the full amplitude to any desired order of expansion in the underlying hadronic two-body amplitude. Moreover, it allows for the approximate incorporation of all neglected higher-order mechanisms in terms of a phenomenological remainder current. The effect and phenomenological usefulness of this remainder current is assessed in a tree-level calculation of the $\gamma N \to K K \Xi$ reaction., Comment: 33 pages, 34 figures

In the light baryon sector resonances can be broad and overlapping and are in most cases not directly visible in the cross section data. Automatized model selection techniques that introduce penalties for resonances can be used to determine the minimally needed set of resonances to describe the data. Several possible penalization schemes are compared. As an application we perform a blindfold identification of hyperon resonances in the $\bar{K} N \to K \Xi$ reaction based on the Least Absolute Shrinkage and Selection Operator (LASSO) in combination with the Bayesian Information Criterion (BIC). We find ten resonances --- out of the 21 above-threshold hyperon resonances with spin $J\le 7/2$ listed by the Particle Data Group. In traditional analyses, it is practically impossible to test the relevance of all resonances and their combinations that may potentially contribute to the reaction. By contrast, the present method proves capable of determining the relevant resonances among a large pool of candidates., Comment: 19 pages, 11 figures, version accepted by journal

Quasi-free photoproduction of $ \eta{^\prime}$ -mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel - TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production threshold up to 2.5GeV. The $ \eta{^\prime}$ -mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the $ \gamma$ n $ \rightarrow$ n $ \eta{^\prime}$ reaction. In addition, also first estimates for coherent $ \eta{^\prime}$ -production off the deuteron have been obtained. In agreement with model predictions, the total cross-section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and t -channel exchanges

We use an effective Lagrangian approach in the tree-level approximation to analyze the high-precision cross-section data for $$\gamma p \rightarrow K^{*+}\Lambda $$ reported by the CLAS Collaboration. Apart from the t-channel K, $$\kappa $$ , $$K^*$$ exchanges, the s-channel nucleon (N) exchange, the u-channel $$\Lambda $$ , $$\Sigma $$ , $$\Sigma ^*(1385)$$ exchanges, and the generalized contact current, we found that at least two nucleon resonances ( $$N^*$$ ’s) should be considered to well describe the high-precision cross-section data. One is the $$N(2060){5/2}^-$$ that is responsible for the shape of the angular distribution near the $$K^*\Lambda $$ threshold via its interference with the t-channel K exchange, and the other could be one of the $$N(2000){5/2}^+$$ , $$N(2040){3/2}^+$$ , $$N(2100){1/2}^+$$ , $$N(2120){3/2}^-$$ and $$N(2190){7/2}^-$$ . The results for $$\Sigma $$ , T and P are predicated and it is pointed out that more data on these observables are needed to further pin down the resonance contents and their associated parameters in this reaction.

The high-precision cross section data for the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{*+}\mathrm{\ensuremath{\Lambda}}$ reported by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility have been analyzed based on an effective Lagrangian approach in the tree-level approximation. Apart from the $t$-channel $K,\phantom{\rule{0.16em}{0ex}}\ensuremath{\kappa},\phantom{\rule{0.16em}{0ex}}{K}^{*}$ exchanges, the $s$-channel nucleon ($N$) exchange, the $u$-channel $\mathrm{\ensuremath{\Lambda}},\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Sigma}},\phantom{\rule{0.16em}{0ex}}{\mathrm{\ensuremath{\Sigma}}}^{*}(1385)$ exchanges, and the generalized contact term, the contributions from the near-threshold nucleon resonances in the $s$ channel are also taken into account in constructing the reaction amplitude. It is found that to achieve a satisfactory description of the differential cross section data, at least two nucleon resonances should be included. By including the $N(2060){5/2}^{\ensuremath{-}}$ resonance, which is responsible for the shape of the angular distribution near the ${K}^{*}\mathrm{\ensuremath{\Lambda}}$ threshold, and one of the $N(2000){5/2}^{+}, N(2040){3/2}^{+},\phantom{\rule{0.16em}{0ex}}N(2100){1/2}^{+},\phantom{\rule{0.16em}{0ex}}N(2120){3/2}^{\ensuremath{-}}$ and $N(2190){7/2}^{\ensuremath{-}}$ resonances, one can describe the cross section data quite well, with the fitted resonance masses and widths compatible with those advocated by the Particle Data Group. The resulted predictions of the beam, target, and recoil asymmetries are found to be quite different from various fits, indicating the necessity of the spin observable data for $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{*+}\mathrm{\ensuremath{\Lambda}}$ to further pin down the resonance contents and associated parameters in this reaction.

A field-theoretical description of the photoproduction of two pions off the nucleon is presented that applies to real as well as virtual photons in the one-photon approximation. The Lorentz-covariant theory is complete at the level of all explicit Faddeev-type three-body final-state mechanisms of dressed interacting hadrons, including those of the nonlinear Dyson-Schwinger type. All electromagnetic currents are constructed to satisfy their respective (generalized) Ward-Takahashi identities and thus satisfy local gauge invariance as a matter of course. The Faddeev-type ordering structure results in a natural expansion of the full two-pion photoproduction current $\Mpp^\mu$ in terms of multiple loops that preserve gauge invariance order by order in the number of loops, which in turn lends itself naturally to practical applications of increasing sophistication with increasing number of loops., Comment: Contributed talk at Few-Body 20 Conference, 20-25 August 2012, Fukuoka, Japan

The photoproduction of cascade baryons off nucleons will be discussed. This is the first theoretical effort in connection to the cascade baryon spectroscopy program initiated recently at the Thomas Jefferson National Laboratory. Specifically, the reaction γ N → K K Ξ is studied within a relativistic meson exchange model of hadronic interactions. The basic features of this reaction and their manifestations in some of the observables are presented.

It is shown that the relative sign between the NNω and πρω coupling constants can be determined most sensitively from ω production processes in NN collisions. Recent data on these reactions clearly favor the sign of the πρω coupling constant which is opposite to that inferred from studies of the photoproduction reaction in combination with the vector meson dominance assumption and used by many authors. Implication of this finding in the description of other reactions is discussed.

The $\overline{K}+N\ensuremath{\rightarrow}K+\mathrm{\ensuremath{\Xi}}$ reaction is studied for center-of-momentum energies ranging from threshold to 3 GeV in an effective Lagrangian approach that includes the hyperon $s$- and $u$-channel contributions as well as a phenomenological contact amplitude. The latter accounts for the rescattering term in the scattering equation and possible short-range dynamics not included explicitly in the model. Existing data are well reproduced and three above-the-threshold resonances were found to be required to describe the data, namely, the $\mathrm{\ensuremath{\Lambda}}(1890), \mathrm{\ensuremath{\Sigma}}(2030)$, and $\mathrm{\ensuremath{\Sigma}}(2250)$. For the latter resonance we have assumed the spin-parity of ${J}^{P}=5/{2}^{\ensuremath{-}}$ and a mass of 2265 MeV. The $\mathrm{\ensuremath{\Sigma}}(2030)$ resonance is crucial in achieving a good reproduction of not only the measured total and differential cross sections but also the recoil polarization asymmetry. More precise data are required before a more definitive statement can be made about the other two resonances, in particular, about the $\mathrm{\ensuremath{\Sigma}}(2250)$ resonance that is introduced to describe a small bump structure observed in the total cross section of ${K}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{K}^{+}+{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}$. The present analysis also reveals a peculiar behavior of the total cross-section data in the threshold energy region in ${K}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{K}^{+}+{\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}$, where the $P$ and $D$ waves dominate instead of the usual $S$ wave. Predictions for the target-recoil asymmetries of the $\overline{K}+N\ensuremath{\rightarrow}K+\mathrm{\ensuremath{\Xi}}$ reaction are also presented.

D. Ronchen, M. Doring, F. Huang, H. Haberzettl, J. Haidenbauer, C. Hanhart, S. Krewald, U.-G. Meisner, and K. Nakayama 1 Institut fur Kernphysik and Julich Center for Hadron Physics, Forschungszentrum Julich, 52425 Julich, Germany 2 Helmholtz-Institut fur Strahlenund Kernphysik (Theorie) and Bethe Center for Theoretical Physics, Universitat Bonn, Nusallee 14-16, 53115 Bonn, Germany 3 Institute for Nuclear Studies and Department of Physics, The George Washington University, Washington, DC 20052, USA 4 School of Physics, University of Chinese Academy of Sciences, Huairou District, Beijing 101408, China 5 Department of Physics and Astronomy, University of Georgia, Athens, Georgia 30602, USA 6 Institute for Advanced Simulation, Forschungszentrum Julich, 52425 Julich, Germany

Recent developments in the search of exotic pentaquark hadrons are briefly reviewed. We then focus on investigating how the exotic pentaquark Θ ( 1540 ) baryon production can be identified in the γ N → K K ¯ N reactions, focusing on the influence of the background (non- Θ production) mechanisms. By imposing the SU(3) symmetry and using various quark model predictions, we are able to fix the coupling constants for evaluating the kaon backgrounds, the K K ¯ production through the intermediate vector meson and tensor meson photoproduction, and the mechanisms involving intermediate Λ ( 1116 ) , Λ ( 1405 ) , Λ ( 1520 ) , Σ ( 1193 ) , Σ ( 1385 ) , and Δ ( 1232 ) states. The vector meson photoproduction part is calculated from a phenomenological model which describes well the experimental data at low energies. We point out that the neutral tensor meson production cannot be due to π 0 -exchange as done by Dzierba et al. [Phys. Rev. D 69 (2004) 051901] because of C parity. The neutral tensor meson production is estimated by considering the vector meson exchange and found to be too weak to generate any peak at the position near Θ ( 1540 ) . For Θ ( 1540 ) production, we assume that it is an isoscalar and hence can only be produced in γ n → K + K - n and γ p → K 0 K ¯ 0 p reactions, but not in γ p → K + K - p and γ n → K 0 K ¯ 0 n . The total cross section data of γ p → K + K - p is thus used to fix the form factors which regularize the background amplitudes so that the signal of Θ ( 1540 ) in γ n → K + K - n and γ p → K 0 K ¯ 0 p cross sections can be predicted. We find that the predicted K + K - and K + n invariant mass distributions of the γ n → K + K - n reaction can qualitatively reproduce the shapes of the JLab data. However, the predicted Θ ( 1540 ) peak cannot be identified unambiguously with the data. High statistics experiments are needed to resolve the problem. We also find that an even-parity Θ is more likely to be detected, while it will be difficult to identify an odd-parity Θ , even if it exists, from the background continuum, if its coupling constants are small as in the present quark model predictions.

Various model-independent aspects of the $\overline{K}N\ensuremath{\rightarrow}K\ensuremath{\Xi}$ reaction are investigated, starting from the determination of the most general structure of the reaction amplitude for $\ensuremath{\Xi}$ baryons with ${J}^{P}={\frac{1}{2}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${\frac{3}{2}}^{\ifmmode\pm\else\textpm\fi{}}$ and the observables that allow a complete determination of these amplitudes. Polarization observables are constructed in terms of spin-density matrix elements. Reflection symmetry about the reaction plane is exploited, in particular, to determine the parity of the produced $\ensuremath{\Xi}$ in a model-independent way. In addition, extending the work of Biagi et al. [Z. Phys. C 34, 175 (1987)], a way of determining simultaneously the spin and parity of the ground state of $\ensuremath{\Xi}$ baryons as well as those of the excited $\ensuremath{\Xi}$ states is presented.

In the present work, we investigate subsequential production of three kaons and $\Omega^-$ baryon based on an effective Lagrangian approach. We only consider the intermediate states with the light mass baryon to suggest the minimum of the total cross section. Coupling constants for verteces of meson-octet baryons are fixed from the empirical data and/or quark models together with SU(3) symmetry considerations and these for meson-decouplet are predicted not only quark model but also Chiral-quark soliton model calculation. Gauge invariance of the resulting amplitude is maintained by introducing the contact currents by extending the gauge-invariant approach of Haberzettl for one-meson photoproduction to two-meson photoproduction., Comment: 4 pages, 2 figures, XV International Conference on Hadron Spectroscopy-Hadron 2013, 4-8 November 2013, Nara, Japan

The production of $\ensuremath{\omega}$ mesons in proton-proton collisions for proton incident energies up to $2.2 \mathrm{GeV}$ is investigated within a meson-exchange model of hadronic interactions. We find a large cancellation between the dominant $\ensuremath{\pi}\ensuremath{\rho}\ensuremath{\omega}$ meson-exchange current and nucleonic current contributions. A comparison with preliminary data from SATURNE calls for the inclusion of off-shell form factors at the $\mathrm{NN}\ensuremath{\omega}$ and $\ensuremath{\pi}\ensuremath{\rho}\ensuremath{\omega}$ production vertices. Due to the present lack of knowledge of these form factors, together with the destructive interference mentioned above, the relative magnitude of the nucleonic and meson-exchange current contributions cannot be determined from existing total cross section data. However, it is shown that the angular distribution of the produced $\ensuremath{\omega}$ mesons provides a unique and clear signature of the magnitude of these currents, thus allowing one to disentangle these two basic reaction mechanisms.

Elastic πN scattering and the world data of the family of reactions π − p → ηn, K 0 Λ, K 0 Σ 0, K + Σ −, and π + p → K + Σ + are described simultaneously in an analytic, unitary, coupled-channel approach. SU(3) flavor symmetry is used to relate both the t - and the u - channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. Partial-wave amplitudes are determined and the resonance content is extracted from the analytic continuation, including resonance positions and branching ratios, and possible sources of uncertainties are discussed. The results provide the final-state interactions for the ongoing analysis of photo- and electroproduction data.

The meson production and associated excited-baryon-analysis program by the Ju

We investigate the effect of meson-decay diagrams on the proton-proton bremsstrahlung process. We explicitly include short-range correlations by calculating single- and double-scattering diagrams using an NN T-matrix interaction. We find that in general these diagrams interfere destructively with the corresponding Born amplitudes so that the net effect of the meson-decay process on proton-proton bremsstrahlung reactions is considerably smaller than that of the Born amplitude alone.

To understand the structure of hadrons and their production mechanisms, it is very important to be able to identify their quantum numbers. For investigating the spin and parity quantum numbers of $\Xi$ and $\Omega$ baryons, in particular, it is very important to understand strong interactions in the strangeness sector. At present, such quantum numbers are known for only a few of the $\Xi$ baryons, and even the parity of the ground state $\Xi(1318)$ has never been measured. In this article, we present a novel, model-independent way to determine the parity of $\Xi$ baryons solely based on symmetry considerations for the hadronic reaction $\bar{K} N \to K \Xi$., Comment: 4 pages, Presented at the 20th International IUPAP Conference on Few-Body Problems in Physics, 20 - 25 August, 2012, Fukuoka, Japan

A complete, rigorous relativistic field-theory formulation of the nucleon-nucleon ($\mathit{NN}$) bremsstrahlung reaction is presented. The resulting amplitude is analytic, Lorentz covariant and unitary as a matter of course and it is gauge invariant; i.e., it satisfies a generalized Ward-Takahashi identity. The novel feature of this approach is the consistent microscopic implementation of local gauge invariance across all interaction mechanisms of the hadronic systems, thus serving as a constraint for all subprocesses. The formalism is quite readily adapted to approximations and thus can be applied even in cases where the microscopic dynamical structure of the underlying interacting hadronic systems is either not known in detail or too complex to be treated in detail. We point out how the interaction currents resulting from the photon being attached to nucleon-nucleon-meson vertices can be treated by phenomenological four-point contact currents that preserve gauge invariance. In an advance application of the present formalism [Nakayama and Haberzettl, Phys. Rev. C 80, 051001(R) (2009)], such interaction currents were found to contribute significantly in explaining experimental data. In addition, we provide a scheme that permits---through an introduction of phenomenological five-point contact currents---the approximate treatment of current contributions resulting from pieces of the $\mathit{NN}$ interaction that cannot be incorporated exactly. In each case, the approximation procedure ensures gauge invariance of the entire bremsstrahlung amplitude. We also discuss the necessary modifications when taking into account baryonic states other than the nucleon $N$; in detail, we consider the $\ensuremath{\Delta}(1232)$ resonance by incorporating the couplings of the $\mathit{NN}$ to the $N\ensuremath{\Delta}$ and $\ensuremath{\Delta}\ensuremath{\Delta}$ systems and the $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\Delta}$ transitions.

The charged and neutral pion photoproduction reactions are investigated in a dynamical coupled-channels approach based on the formulation of Haberzettl, Huang, and Nakayama [Phys. Rev. C 83, 065502 (2011)]. The hadronic final-state interaction is provided by the Juelich pi-N model, which includes the channels pi-N and eta-N comprising stable hadrons as well as the effective pi-pi-N channels pi-Delta, sigma-N, and rho-N. This hadronic model has been quite successful in describing pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. By construction, the full pion photoproduction current satisfies the generalized Ward-Takahashi identity and thus is gauge invariant as a matter of course. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to pi+ n, gamma p to pi0 p, gamma n to pi- p and gamma n to pi0 n are in good agreement with the experimental data., Comment: revtex4-1, 26 pages, 6 tables, 21 figures

Based on reflection symmetry in the reaction plane, it is shown that measuring the transverse spin-transfer coefficient $K_{yy}$ in the $\bar{K}N \to K\Xi$ reaction directly determines the parity of the produced cascade hyperon in a model-independent way as $\pi_\Xi =K_{yy}$, where $\pi_\Xi =\pm 1$ is the parity. This result based on Bohr's theorem provides a completely general, universal relationship that applies to the entire hyperon spectrum. A similar expression is obtained for the photoreaction $\gamma N \to K K \Xi$ by measuring both the double-polarization observable $K_{yy}$ and the photon-beam asymmetry $\Sigma$. Regarding the feasibility of such experiments, it is pointed out that the self-analyzing property of the $\Xi$'s can be invoked, thus requiring only a polarized nucleon target., Comment: 4 pages, REVTeX, to be published in Phys. Rev. C

The photoproduction of pion off nucleon is investigated within a dynamical coupled-channels approach based on the Juelich pi-N model, which has been quite successful in the description of pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to pi^+ n, gamma p to pi^0 p and gamma n to pi^- p are in good agreement with the experimental data., Comment: Talk given at The 8th International Workshop on the Physics of Excited Nucleons (NSTAR2011), May 17-20, 2011, Thomas Jefferson National Accelerator Facility, Newport News, Virginia, USA

The pp -> pp gamma reaction has been studied at a beam energy of 310 MeV by detecting both final protons at the PROMICE-WASA facility and identifying the photon through the resulting missing-mass peak. The photon angular distribution in the center-of-mass system and those of the proton-proton relative momentum with respect to the beam direction and to that of the recoil photon were determined reliably up to a final pp excitation energy of E-pp similar to 30 MeV. Except for very small E-pp values, the behavior of these distributions with excitation energy is well reproduced by a new refined model of the hard bremsstrahlung process. The model reproduces absolutely the total cross section and its energy dependence to within the experimental and theoretical uncertainties.

The recent data taken by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility for the reaction of {gamma}p{yields}K{sup +}K{sup +}{Xi}{sup -} are reanalyzed within a relativistic meson-exchange model of hadronic interactions. The present model is an extension of the one developed in an earlier work by Nakayama, Oh, and Haberzettl [Phys. Rev. C 74, 035205 (2006)]. In particular, the role of the spin-5/2 and -7/2 hyperon resonances, which were not included in the previous model, is investigated in the present study. It is shown that the contribution of the {Sigma}(2030) hyperon having spin-7/2 and positive parity has a key role to bring the model predictions into a fair agreement with the measured data for the K{sup +}{Xi}{sup -} invariant mass distribution.

A field-theory-based approach to pion photoproduction off the nucleon is used to derive a microscopically consistent formulation of the fully dressed electromagnetic nucleon current in an effective Lagrangian formalism. It is shown how the rigorous implementation of local gauge invariance at all levels of the reaction dynamics provides equations that lend themselves to practically manageable truncations of the underlying nonlinearities of the problem. The requirement of consistency also suggests a novel way of treating the pion photoproduction problem. Guided by a phenomenological implementation of gauge invariance for the truncated equations that has proved successful for pion photoproduction, an expression for the fully dressed nucleon current is given that satisfies the Ward-Takahashi identity for a fully dressed nucleon propagator as a matter of course. Possible applications include meson photo- and electroproduction processes, bremsstrahlung, Compton scattering, and $ee'$ processes off nucleons., 10 pages, 9 figures

The nucleon-nucleon bremsstrahlung reaction is investigated based on a fully gauge-invariant relativistic meson-exchange model approach. To account consistently for the complicated part of the interaction current (which at present is too demanding to be calculated explicitly), a generalized contact current is introduced following the approach of H. Haberzettl, K. Nakayama, and S. Krewald [Phys. Rev. C 74, 045202 (2006)]. The contact interaction current is constructed phenomenologically such that the resulting full bremsstrahlung amplitude satisfies the generalized Ward-Takahashi identity. The formalism is applied to describe the high-precision proton-proton bremsstrahlung data at 190 MeV obtained at KVI [H. Huisman et al., Phys. Rev. C 65, 031001(R) (2002)]. The present results show good agreement with the data, thus removing the long-standing discrepancy between the theoretical predictions and experimental data. The present investigation, therefore, points to the importance of properly taking into account the interaction current for this reaction.

The reaction mechanisms for $K\ensuremath{\Sigma}(1385)$ photoproduction from the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\Sigma}}^{0}(1385)$ in the resonance energy region are investigated in a hadronic model. Both contributions from $N$ and \ensuremath{\Delta} resonances of masses around 2 GeV as given in the Review of Particle Data Group and by the quark model predictions are included. The Lagrangians for describing the decays of these resonances into $K\ensuremath{\Sigma}(1385)$ are constructed with the coupling constants determined from the decay amplitudes predicted by a quark model. Comparing the resulting total cross section for the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}{\ensuremath{\Sigma}}^{0}(1385)$ with the preliminary data from the Thomas Jefferson National Accelerator Facility, we find that the most important contributions are from the two-star-rated resonances $\ensuremath{\Delta}(2000){F}_{35},\ensuremath{\Delta}(1940){D}_{33}$, and $N(2080){D}_{13}$, as well as the missing resonance $N{\frac{3}{2}}^{\ensuremath{-}}(2095)$ predicted in the quark model. Predictions on the differential cross section and photon asymmetry in this reaction are also given.

The $\eta$-meson production in photon- and hadron-induced reactions, namely, $\gamma p \to p \eta$, $\pi^- p \to n \eta$, $pp \to pp\eta$, and $pn \to pn\eta$, are investigated in a combined analysis in order to learn about the relevant production mechanisms and the possible role of nucleon resonances in these reactions. We consider the nucleonic, mesonic, and nucleon resonance currents constructed within an effective Lagrangian approach and compare the results with the available data for cross sections and spin asymmetries for these reactions. We found that the reaction $\gamma p \to p \eta$ could be described well with the inclusion of the well-established $S_{11}(1535)$, $S_{11}(1650)$, $D_{13}(1520)$, and $D_{13}(1700)$ resonances, in addition to the mesonic current. Consideration of other well-established resonances in the same mass region, including the spin-5/2 resonances, $D_{15}(1675)$ and $F_{15}(1680)$, does not further improve the results qualitatively. For the reaction $\pi^- p \to n \eta$, the $P_{13}(1720)$ resonance is found to be important for reproducing the structure observed in the differential cross section data. Our model also improves the description of the reaction $NN \to NN\eta$ to a large extent compared to the earlier results by Nakayama \textit{et al.} [Phys.\ Rev.\ C \textbf{68}, 045201 (2003)]. For this reaction, we address two cases where either the $S_{11}(1535)$ or the $D_{13}$ dominates. Further improvement in the description of these reactions and the difficulty to uniquely determine the nucleon resonance parameters in the present type of analysis are discussed., Comment: 25 pages, 15 figures, REVTeX, figures and references are added

We investigate the reaction of $\gamma N \to K \bar{K} N$ focusing on the photoproduction of $\Lambda(1520)$ and putative exotic $\Theta(1540)$. We consider various background production mechanisms including the production of vector mesons, tensor mesons, and other $\Lambda$ and $\Sigma$ hyperons. We discuss the angular distribution of $\Lambda(1520)$ photoproduction cross section and the radiative decays of $\Lambda(1520)$. We also discuss what we expect for the invariant mass distributions if the $\Theta(1540)$ is formed in the reaction, with the parameters studied so far. We find that the peak in the $KN$ invariant mass distribution, if confirmed, can hardly come from the kinematic reflections, especially, due to the tensor meson backgrounds.

The recent high-precision data for the reaction $\gamma p\to p\eta'$ at photon energies in the range 1.5--2.3 GeV obtained by the CLAS collaboration at the Jefferson Laboratory have been analyzed within an extended version of the photoproduction model developed previously by the authors based on a relativistic meson-exchange model of hadronic interactions [Phys. Rev. C \textbf{69}, 065212 (2004)]. The $\eta'$ photoproduction can be described quite well over the entire energy range of available data by considering $S_{11}$, $P_{11}$, $P_{13}$, and $D_{13}$ resonances, in addition to the $t$-channel mesonic currents. The observed angular distribution is due to the interference between the $t$-channel and the nucleon $s$- and $u$-channel resonance contributions. The $j=3/2$ resonances are required to reproduce some of the details of the measured angular distribution. For the resonances considered, our analysis yields mass values compatible with those advocated by the Particle Data Group. We emphasize, however, that cross-section data alone are unable to pin down the resonance parameters and it is shown that the beam and/or target asymmetries impose more stringent constraints on these parameter values. It is found that the nucleonic current is relatively small and that the $NN\eta^\prime$ coupling constant is not expected to be much larger than 2.

The photoproduction reaction $\gamma N \to K K \Xi$ is investigated based on a relativistic meson-exchange model of hadronic interactions. The production amplitude is calculated in the tree-level approximation from relevant effective Lagrangians, whose (coupling constant) parameters are mostly fixed from the empirical data and/or quark models together with SU(3) symmetry considerations. Gauge invariance of the resulting amplitude is maintained by introducing the contact currents by extending the gauge-invariant approach of Haberzettl for one-meson photoproduction to two-meson photoproduction. The role of the intermediate low-lying hyperons and of the intermediate higher-mass hyperon resonances are analyzed in detail. In particular, the basic features of the production of $\Xi^-(1318)$ in $\gamma p \to K^+ K^+ \Xi^-$ and their possible manifestations in the forthcoming experimental data are discussed., Comment: 18 pages, 17 figures, REVTeX, 1 figure added, to appear in Phys. Rev. C

We analyze the problem of how to determine the parity of the ${\ensuremath{\Theta}}^{+}$ pentaquark in the reaction $\ensuremath{\gamma}N\ensuremath{\rightarrow}K\ensuremath{\Theta}\ensuremath{\rightarrow}\mathrm{NK}\overline{K}$, where $N=n,p$. Our model calculations indicate that the contribution of the nonresonant background of the reaction $\ensuremath{\gamma}N\ensuremath{\rightarrow}\mathrm{NK}\overline{K}$ cannot be neglected and that suggestions to determine the parity based solely on the initial-stage process $\ensuremath{\gamma}N\ensuremath{\rightarrow}K\ensuremath{\Theta}$ cannot be implemented cleanly. We discuss the various mechanisms that contribute to the background. Among the observables considered in this work, the triple spin asymmetry shows a kinematical ``window'' where the predictions are sensitive to the ${\ensuremath{\Theta}}^{+}$ parity and insensitive to the various reaction mechanisms considered, and thus, this observable offers a possibility of determining the parity of the ${\ensuremath{\Theta}}^{+}$.

Spin observables in the photoproduction of the $\Theta^+$ are explored for the purpose of determining the parity of the $\Theta^+$. Based on reflection symmetry in the scattering plane, we show that certain spin observables in the photoproduction of the $\Theta^+$ can be related directly to its parity. We also show that measurements of both the target nucleon asymmetry and the $% \Theta^+$ polarization may be useful in determining the parity of $\Theta^+$ in a model-independent way. Furthermore, we show that no combination of spin observables involving only the polarization of the photon and/or nucleon in the initial state can determine the parity of $\Theta^+$ unambiguously., Comment: LaTeX, 11 pages, minor revision

We critically examine the possibility of determining the parity of the $\Theta^+(1540)$ from the reactions $NN\to \Theta^+Y$ ($Y$ = $\Lambda$, $\Sigma$) recently discussed in the literature. Specifically, we study the energy dependence of those observables that were suggested to be the most promising ones to unravel the parity of the $\Theta^+$, namely the spin correlation coefficient $A_{xx}$, and the spin transfer coefficient $D_{xx}$. We show that the energy dependence of $\sigma_0(1+A_{xx})$, corresponding to the spin-triplet production cross section, guarantees unambiguous information on the parity of the $\Theta^+$. Here, $\sigma_0$ denotes the unpolarized cross section. Also, the possibility to determine the parity of the $\Theta^+$ through the energy dependence of $\sigma_0D_{xx}$ is discussed., Comment: 13 pages, 6 figures

The most general spin structures of the spin-1/2 baryon and spinless meson production operator for both photon and nucleon induced reactions are derived from the partial-wave expansions of these reaction amplitudes. The present method provides the coefficients multiplying each spin operator in terms of the partial-wave matrix elements. The result should be useful in studies of these reactions based on partial-wave analyses, especially, when spin observables are considered., RevTex 34 pages, revised version

It is shown that most of the available data on the $pp\to pp\eta$ reaction, including the invariant mass distributions in the $pp\to pp\eta$ reaction recently measured at COSY, can be understood in terms of the partial-wave amplitudes involving final $pp$ $S$ and $P$ states and the $\eta$ meson s-wave. This finding, together with the fact that results within a meson--exchange model are especially sensitive to the details of the excitation mechanism of the $S_{11}(1535)$ resonance, demonstrates the possibility of investigating the properties of this resonance in $NN$ collisions. The spin correlation function $C_{xx}$ is shown to disentangle the $S$- and $P$-wave contributions. It is also argued that spin correlations may be used to help constrain the contributions of the amplitudes corresponding to the final $pp$ $^3P_0$

We present results for the $p p \to p p \omega$ reaction studied by considering two different scenarios: with and without the inclusion of nucleon resonance excitations. The recently measured angular distribution by the COSY-TOF Collaboration at an excess energy of $Q = 173$ MeV and the energy dependence of the total cross section data for $\pi^- p \to \omega n$ are used to calibrate the model parameters. The inclusion of nucleon resonances improves the theoretical prediction for the energy dependence of the total cross section in $pp \to pp\omega$ at excess energies $Q < 31$ MeV. However, it still underestimates the data by about a factor of two, and remains a problem in understanding the reaction mechanism., Comment: Fig.5 and text modified, Latex, 4 pages, 8 embedded figures, uses espcrc1.sty (included), talk presented at PANIC02, Osaka, Japan, 30 September - 4 October 2002

$\ensuremath{\eta}$ meson production in both proton-proton and proton-neutron collisions is investigated within a relativistic meson exchange model of hadronic interactions. It is found that the available cross section data can be described equally well by either the vector or pseudoscalar meson exchange mechanism for exciting the ${S}_{11}(1535)$ resonance. It is shown that the analyzing power data can potentially be very useful in distinguishing these two scenarios for the excitaion of the ${S}_{11}(1535)$ resonance.

In preparation for forthcoming experiments on multi-strangeness baryon pro- duction at JPARC, we analyze the general features of Ξ production in antikaon-induced reactions. A simple model is applied to this reaction; problems with retaining s-u sym- metry are addressed with a generalized contact term. Existing data are reproduced and any hyperon resonance features are extracted.

In anticipation of the multi-strangeness physics program planned at major laboratories — such as J-PARC, JLab and SPring-8 — we investigate the Ξ resonance production off the nucleon in [Formula: see text]- and γ-induced reactions.

The pp -> pp eta' reaction is investigated within a relativistic meson-exchange model of hadronic interactions. We explore the role of nucleonic and mesonic, as well as the N* resonance currents, in producing eta' mesons. In order to learn more about the production mechanisms, new measurements in the energy region far from the threshold are required., 35 pages including 9 figures

A combined analysis of the free and quasi-free γN → η ′N , NN → NN η ′, and πN → η ′N reactions has been performed within an effective Lagrangian approach. It is 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 η ′ production processes in the resonance-energy region up to √s = 2.35 GeV.