Search

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

The high-precision cross-section data for the reaction $\gamma p \to K^{*+}\Lambda$ 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 $K$, $\kappa$, $K^*$ exchanges, the $s$-channel nucleon ($N$) exchange, the $u$-channel $\Lambda$, $\Sigma$, $\Sigma^*(1385)$ 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 $N(2060){5/2}^-$ resonance, which is responsible for the shape of the angular distribution near the $K^*\Lambda$ threshold, and one of the $N(2000){5/2}^+$, $N(2040){3/2}^+$, $N(2100){1/2}^+$, $N(2120){3/2}^-$ and $N(2190){7/2}^-$ 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 $\gamma p \to K^{*+}\Lambda$ to further pin down the resonance contents and associated parameters in this reaction., Comment: 19 pages, 14 figures

We compare the methods of amplitude reconstruction, for a complete experiment and a truncated partial-wave analysis, applied to the electroproduction of pseudoscalar mesons. We give examples which show, in detail, how the amplitude reconstruction (observables measured at a single energy and angle) is related to a truncated partial-wave analysis (observables measured at a single energy and a number of angles). A connection is made to existing data., Comment: 17 pages, 2 figures

We compare the methods of amplitude reconstruction, for a complete experiment and a truncated partial wave analysis, applied to the photoproduction of pseudo-scalar mesons. The approach is pedagogical, showing in detail how the amplitude reconstruction (observables measured at a single energy and angle) is related to a truncated partial-wave analysis (observables measured at a single energy and a number of angles)., Comment: 17 pages

The KL2016 Workshop is following the Letter of Intent LoI12-15-001 "Physics Opportunities with Secondary KL beam at JLab" submitted to PAC43 with the main focus on the physics of excited hyperons produced by the Kaon beam on unpolarized and polarized targets with GlueX setup in Hall D. Such studies will broaden a physics program of hadron spectroscopy extending it to the strange sector. The Workshop was organized to get a feedback from the community to strengthen physics motivation of the LoI and prepare a full proposal. Further details about the Workshop can be found on the web page of the conference: http://www.jlab.org/conferences/kl2016/index.html ., Comment: 238 pages, Editors: M. Amaryan, E. Chudakov, C. Meyer, M. Pennington, J. Ritman, and I. Strakovsky

The $\eta N$ final state is isospin-selective and thus provides access to the spectrum of excited nucleons without being affected by excited $\Delta$ states. To this end, the world database on eta photoproduction off the proton up to a center-of-mass energy of $E\sim 2.3$ GeV is analyzed, including data on differential cross sections, and single and double polarization observables. The resonance spectrum and its properties are determined in a combined analysis of eta and pion photoproduction off the proton together with the reactions $\pi N\to \pi N$, $\eta N$, $K\Lambda$ and $K\Sigma$. For the analysis, the so-called J\"ulich coupled-channel framework is used, incorporating unitarity, analyticity, and effective three-body channels. Parameters tied to photoproduction and hadronic interactions are varied simultaneously. The influence of recent MAMI $T$ and $F$ asymmetry data on the eta photoproduction amplitude is discussed in detail., Comment: 21 pages, 10 figures. Updated to the published version

The $\bar{K} + N \to K + \Xi$ 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 $\Lambda(1890)$, $\Sigma(2030)$, and $\Sigma(2250)$. For the latter resonance we have assumed the spin-parity of $J^P=5/2^-$ and a mass of 2265 MeV. The $\Sigma(2030)$ 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 $\Sigma(2250)$ resonance that is introduced to describe a small bump structure observed in the total cross section of $K^- + p \to K^+ + \Xi^-$. The present analysis also reveals a peculiar behavior of the total cross section data in the threshold energy region in $K^- + p \to K^+ + \Xi^-$, where the $P$- and $D$-waves dominate instead of the usual $S$-wave. Predictions for the target-recoil asymmetries of the $\bar{K} + N \to K + \Xi$ reaction are also presented., Comment: 19 pages, REVTeX, 12 figures, revised version

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 reactions $\gamma p\to\pi^0 p$ and $\gamma p\to\pi^+ n$ are analyzed in a semi-phenomenological approach up to $E\sim2.3$ GeV. Fits to differential cross section and single and double polarization observables are performed. A good overall reproduction of the available photoproduction data is achieved. The J\"ulich2012 dynamical coupled-channel model -which describes elastic $\pi N$ scattering and the world data base of the reactions $\pi N\to\eta N$, $K\Lambda$, and $K\Sigma$ at the same time - is employed as the hadronic interaction in the final state. The framework guarantees analyticity and, thus, allows for a reliable extraction of resonance parameters in terms of poles and residues. In particular, the photocouplings at the pole can be extracted and are presented., Comment: 37 pages, 31 figures. Angles of the photocouplings at the pole adapted to the convention of Phys. Rev. C 87, 068201 (2013)

The mini-proceedings of the Workshop on PWA tools in Hadronic Spectroscopy held in Mainz from February 18th to 20th, 2013., Comment: 55 pages, 23 contributions. Editors: M. Ostrick, M. Fritsch, L. Tiator, and P. Masjuan

Elastic piN scattering and the world data of the family of reactions pi^- p --> eta n, K^0 Lambda$, K^0 Sigma^0, K^+ Sigma^-, and pi^+ p --> K^+ Sigma^+ 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., Comment: 53 pages, 48 figures; results updated to published version

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., Comment: revtex4-1, 2 columns, 22 pages, 17 figures; final version published in PRC

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 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

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

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., Comment: 10 pages, 9 figures

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 Epp ~ 30 MeV. Except for very small Epp 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., Comment: 12 pages with 11 figures. Presentation modified in the light of referee comments

A complete, rigorous relativistic field-theory formulation of the nucleon-nucleon (NN) bremsstrahlung reaction is presented. The resulting amplitude is 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 [K. Nakayama and H. Haberzettl, PRC 80, 051001(R) (2009)], such interaction currents were found to contribute significantly in explaining experimental data. In addition, we provide a scheme that permits the approximate treatment of current contributions resulting from pieces of the 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 $Delta(1232)$ resonance by incorporating the couplings of the $NN$ to the $N\Delta$ and $\Delta\Delta$ systems, and the $\gamma N \to \Delta$ transitions. We apply the formalism to the 280-MeV bremsstrahlung data from TRIUMF [PRD 41, 2689 (1990)] incorporating $\gamma N\Delta$ transition currents and find good agreement with the data., Comment: 17 pages, 11 figures

Pion photoproduction reactions are investigated in a dynamical coupled-channel approach based on the Juelich pi-N model, which presently includes the hadronic pi-N and eta-N stable channels as well as the pi-Delta, sigma-N and rho-N effective channels. This model 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 pi0 p and gamma n to pi- p are in good agreement with the experimental data., Comment: Invited talk given at 12th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon (MENU2010), Williamsburg, USA, May 31-June 4 2010

The nucleon-nucleon bremsstrahlung reaction is investigated based on a fully gauge-invariant relativistic meson-exchange model approach. In order 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 Haberzettl, Nakayama, and Krewald [Phys. Rev. C74, 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. C65, 031001(R) (2002)]. The present results show good agreement with the data, thus removing the longstanding 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., Comment: Minor revisions; as published in Phys. Rev. C

The production of $\eta$ and $\eta^\prime$ mesons in photon- and hadron-induced reactions are studied in a combined analysis based on an effective Lagrangian approach in order to learn about the relevant production mechanisms and the possible role of nucleon resonances in these reactions., Comment: 8 pages, Presented at the International Symposium on Meson Physics, Oct. 1-4, 2008, Krakow, Poland

We report on our recent evaluation of the s-channel cut contribution to J/psi hadroproduction. We show that it is likely significantly larger than the usual cut contribution of the colour-singlet model (CSM), which is known to underestimate the experimental measurements. Here the s-channel cut develops for configurations with off-shell quarks in the bound state. A correct treatment of its contribution requires the introduction of a four point function, partially constrained by gauge invariance and limiting behaviours at small and large momenta. When the unconstrained degrees of freedom are fixed to reproduce the Tevatron data, we show that RHIC data are remarkably well reproduced down to very low transverse momenta P_T without need of resummation of initial-state gluon effects. This unique feature might be typical of s-channel cut contribution., Comment: Presented by J.P. Lansberg at the Joint Meeting Heidelberg-Liege-Paris-Wroclaw (HLPW08), Spa, Belgium, 6-8 March 2008, 10 pages, 8 figures, LaTeX, uses aip-6s.clo, aipproc.cls and aipxfm.sty (included)

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 argue that the s-channel cut contribution to J/psi hadroproduction can be significantly larger than the usual cut contribution of the color-singlet mechanism, which is known to underestimate the experimental measurements. A scenario accounting for intermediate $c\bar(c)$ interactions is proposed that reproduces the data at low- and mid-range transverse momenta P_T from the Fermilab Tevatron and BNL Relativistiv Heavy Ion Collider. The J/psi produced in this manner are polarized predominantly longitudinally., Comment: 4 pages, 4 figures; version (with minor text revisions) as published in PRL

We reexamine the time-delay formalism of Wigner, Eisenbud and Smith, which was developed to analyze both elastic and inelastic resonances. An error in the paper of Smith has propagated through the literature. We correct this error and show how the results of Eisenbud and Smith are related. We also comment on some recent time-delay studies, based on Smith's erroneous interpretation of the Eisenbud result., Comment: 4 pages, no figures

A fully gauge-invariant (pseudoscalar) meson photoproduction amplitude off a nucleon including the final-state interaction is derived. The approach based on a comprehensive field-theoretical formalism developed earlier by one of the authors replaces certain dynamical features of the full interaction current by phenomenological auxiliary contact currents. A procedure is outlined that allows for a systematic improvement of this approximation. The feasibility of the approach is illustrated by applying it to both the neutral and charged pion photoproductions., Comment: 14 pages, 8 figures, RevTeX4; revised version as scheduled to appear in Physical Review C

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

A gauge-invariant formalism is presented for the practical treatment of photo- and electroproduction of pseudoscalar mesons off nucleons that allows an explicit incorporation of hadronic final-state interactions. The semi-phenomenological approach is based on a field theory developed by one of the authors. It generalizes an earlier approach by allowing for systematic improvement of approximations in a controlled manner. The practical feasibility is illustrated by applying the lowest-order result to the photoproduction of both neutral and charged pions., Comment: Plenary talk given at the N*2005 Workshop (Oct. 2005, Tallahassee, FL); to appear in the Proceedings (to be publ. by WorldScientific)

A combined analysis of the existing data on the reactions $\gamma p \to p \eta^\prime $ and $pp \to pp\eta^\prime $, based on a relativistic meson exchange model of hadronic interactions, is presented., Comment: Talk given at the N*2005 Workshop (Oct. 2005, Tallahassee, FL); to appear in the Proceedings (to be publ. by WorldScientific)

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., Comment: Revised version based on revised (finalized) CLAS data (14 pages, 10 figures, RevTeX4)

We analyze the problem of how to determine the parity of the $\Theta^+$ pentaquark in the reaction $\gamma N\to K\Theta\to NK\bar{K}$, where $N=n,p$. Our model calculations indicate that the contribution of the non-resonant background of the reaction $\gamma N\to NK\bar{K}$ cannot be neglected, and that suggestions to determine the parity based solely on the initial-stage process $\gamma N\to K\Theta$ cannot be implemented cleanly. We discuss the various mechanisms that contribute to the background, and we identify some spin observables which are sensitive., Comment: 39 pages, 25 figures

The production of $\eta'$ mesons in the reactions $\gamma p\to p\eta'$ and $pp\to pp\eta'$ is described consistently within a relativistic meson exchange model of hadronic interactions. The photoproduction can be described quite well over the entire energy range of available data by considering an $S_{11}$ and a $P_{11}$ resonance, in addition to the $t$-channel mesonic current. The observed angular distribution is due to the interference between the $t$-channel and the nucleon resonance $s$- and $u$-channel contributions. Our analysis yields positions close to 1650 MeV and 1870 MeV for the $S_{11}$ and $P_{11}$ resonances, respectively. We argue that, at present, identifying these states with the known $S_{11}(1650)$ resonance and the missing $P_{11}$ resonance predicted at 1880 MeV, respectively, would be premature. It is found that the nucleonic current is relatively small and that the $NN\eta^\prime$ coupling constant cannot be much larger than $g_{NN\eta^\prime}=3$. As for the $p p \to p p \eta^\prime$ reaction, different current contributions are constrained by a combined analysis of this and the photoproduction reaction. Difficulties to simultaneously account for the 47-MeV and 144-MeV angular distributions measured by the COSY-11 and DISTO collaborations, respectively, are addressed., Comment: minor revision, scheduled to a appear in Phys. Rev. C 69 (May 2004), revtex, 17 pages, 10 figures, 3 tables

It is shown that comments by Guichon [hep-ph/0012126], and also by Bernard, Kaiser, and Mei{\ss}ner [hep-ph/0101062], regarding my recent criticism [Phys. Rev. Lett. 85, 3576 (2000)] of how the axial form factor is supposed to enter pion electroproduction do not address the main point of my argument and therefore are irrelevant., Comment: To appear in Phys. Rev. Lett

Photo- and electroproduction processes of K+ on the deuteron are investigated theoretically. Modern hyperon-nucleon forces as well as an updated kaon production operator on the nucleon are used. Sizable effects of the hyperon-nucleon final state interaction are seen in various observables. Especially the photoproduction double polarization observable C_z is shown to provide a handle to distinguish different hyperon-nucleon force models., Comment: 4 pages, 7 eps-figures, talk given at the VII International Conference on Hypernuclear and Strange Particle Physics, Torino, Italy, October 23-27, 2000, to appear in the proceedings (Nucl. Phys. A)

We review the opportunities and challenges in the field of hypernuclear physics with electromagnetic probes. An overview is presented regarding our current understanding of the elementary production process on the nucleon. This amplitude is then used in the nuclear environment to study the hyperon-nucleon (YN) interaction. We discuss two scenarios: hypernuclear excitation that allows the investigation of hypernuclear structure and the bound Lambda in the nucleus, and quasifree kaon production on the deuteron and on nuclei, which permits a more direct access to the YN force. Specific examples are given for few-body systems and shell-model nuclei., Comment: 23 pages in LaTeX, Keynote talk at the JLab workshop on Hypernuclear Physics with Electromagnetic Probes (HYPLAB99), Jefferson Lab, Dec. 2-4, 1999

Resonances with strong couplings to strangeness channels are investigated in a coupled-channels framework of kaon production with pions and photons. We find evidence for an additional D13 resonance around 1900 MeV that has a significant decay width into the K Lambda final state. Such a state has been predicted in a relativistic quark model at 1960 MeV with significant gamma N and K Lambda branching ratios., Comment: 8 pages, Latex, 5 figures

An effective Lagrangian model in a coupled channels framework is applied to extract nucleon resonance parameters. In the K-matrix approximation, we simultaneously analyze all the available data for the transitions from pi N to five possible meson-baryon final states, pi N, pipi N, eta N, K Lambda, and KSigma, in the energy range from pi N threshold up to W = 2 GeV. In this work, we focus our efforts on the K Sigma channel. In particular, we include a set of Delta resonances around 1900 MeV: the S31(1900), P31(1910), and P33(1920) states. In most cases the parameters of nucleon resonances determined by the fits are consistent with the commonly accepted values in the literature. We find that in the low-energy region, the S11(1650), P11(1710), and P13(1720) states are important for the p(pi-,K0)Sigma0 and p(pi-,K+)Sigma- processes while the P33(1600) and D33(1700) states are significant for the pure I=3/2 reaction p(pi+,K+)Sigma+. In the high energy region, we find a D13 state at 1945 MeV which is important for the p(pi-,K0)Sigma0 reaction while the Delta resonances around 1900 MeV show up significantly in the p(pi-,K+)Sigma- and p(pi+,K+)Sigma+ processes., Comment: 9 Pages, Latex, 3 figures, to be published in the Proceeding of the 18th Indonesian National Physics Symposium

Some applications of the elementary kaon photoproduction process are discussed: the investigation of missing resonances in the p(gamma,K+)Lambda channel, the role of the P_{13}(1720) state in the p(gamma,K0)Sigma+ channel, and the calculation of the Gerasimov-Drell-Hearn sum rule. For the latter, we present an extension of our previous study to higher energies., Comment: 3 pages, contribution to FB2000 (Taiwan) March 2000, to be published in Nucl. Phys. A

We analyze the reaction gamma + d --> K^+ + Lambda (Sigma) + N using the Nijmegen soft core hyperon-nucleon interactions NSC97f and NSC89 and a recently updated production operator for the gamma + N --> K^+ + Lambda(Sigma) processes. Significant effects of the YN final state interaction are found near both the K^+ Lambda N and K^+ Sigma N thresholds., Comment: 5 pages, 4 eps-figures, given at the 2nd KEK-Tanashi International Symposium on "Hadron and Nuclear Physics with Electromagnetic Probes", Oct. 1999, Tokyo

Using an isobar model which can reproduce the existing experimental data of kaon photoproduction on the nucleon we investigate some related phenomenological aspects, i.e. the hadronic form factors, contributions of kaon-hyperon final states to the anomalous magnetic moment of the nucleon, and missing nucleon resonances. By reggeizing the appropriate propagators we extend the model to the higher photon energy regime., Comment: 6 pages, 2 eps-figures, invited talk given at the 2nd KEK-Tanashi International Symposium on "Hadron and Nuclear Physics with Electromagnetic Probes", Oct. 1999, Tokyo

New SAPHIR p(gamma,K+)Lambda total cross section data show a resonance structure at a total c.m. energy around 1900 MeV. We investigate this feature with an isobar model and find that the structure can be well explained by including a new D_{13} resonance at 1895 MeV. Such a state has been predicted by a relativistic quark model at 1960 MeV with significant gamma-N and K-Lambda branching ratios. We demonstrate how the measurement of single and double polarization observables can be used to obtain additional information on this resonance. Using recent (e,e'K+) JLab data from Hall C we extract the electromagnetic form factors of this state., Comment: 10 pages, 7 eps-figures, invited talk at the 2nd ICTP International Conference on Perspectives in Hadronic Physics, Trieste, Italy, 10-14 May 1999

Investigations of the quasifree reaction A$(\gamma, K Y)$B are presented in the distorted wave impulse approximation (DWIA). For this purpose, we present a revised tree-level model of elementary kaon photoproduction that incorporates hadronic form factors consistent with gauge invariance, uses SU(3) values for the Born couplings and uses resonances consistent with multi-channel analyses. The potential of exclusive quasifree kaon photoproduction on nuclei to reveal details of the hyperon-nucleus interaction is examined. Detailed predictions for the coincidence cross section, the photon asymmetry, and the hyperon polarization and their sensitivities to the ingredients of the model are obtained for all six production channels. Under selected kinematics these observables are found to be sensitive to the hyperon-nucleus final state interaction. Some polarization observables are found to be insensitive to distortion effects, making them ideal tools to search for possible medium modifications of the elementary amplitude., Comment: 37 pages, 17 figures, revised version

Nucleon resonances are investigated through the electromagnetic production of K-mesons. We study the kaon photoproduction process at tree-level and compare to a recently developed unitary K-matrix approach. Employing hadronic form factors along with the proper gauge prescription yields suppression of the Born terms and leads a resonance dominated process for both K-Lambda and K-Sigma photoproduction. Using new SAPHIR data we find the K+-Lambda photoproduction to be dominated by the S11(1650) at threshold, with additional contributions from the P11(1710) and P13(1720) states. The K-Sigma channel couples to a cluster of Delta resonances around W = 1900 MeV. We briefly discuss some tantalizing evidence for a missing D13 resonance around 1900 MeV with a strong branching ratio into KLambda channel., Comment: 17 pages, LaTex, 10 figures, to be published in the Proceeding of The Workshop on Electron Nucleus Scattering, Elba, Italy, June 22-26, 1998

The general gauge-invariant photoproduction formalism given by Haberzettl is applied to kaon photoproduction off the nucleon at the tree level, with form factors describing composite nucleons. Numerical results show that this gauge-invariance procedure, when compared to Ohta's, leads to a much improved description of experimental data. Predictions for the new Bonn SAPHIR data for $p(\gamma,K^+)\Lambda$ are given., Comment: 4 pages; Contribution to the Proceedings of Baryons '98, Bonn, September 22-26, 1998

The general gauge-invariant photoproduction formalism given by Haberzettl is applied to kaon photoproduction off the nucleon at the tree level, with form factors describing composite nucleons. We demonstrate that, in contrast to Ohta's gauge-invariance prescription, this formalism allows electric current contributions to be multiplied by a form factor, i.e., they do not need to be treated like bare currents. Numerical results show that Haberzettl's gauge procedure, when compared to Ohta's, leads to much improved $\chi^2$ values. Moreover, predictions for the new Bonn SAPHIR data for $p(\gamma,K^+)\Lambda$ are given., Comment: LaTeX, 4 pages, 3 figures, requires fbssuppl.sty and epsfig.sty. To appear in the "Proceedings of the 16th European Conference on Few-Body Problems in Physics, June 1-6, 1998, Autrans/France", to be published as a Few-Body Systems Supplement

We show how the gauge-invariance formulation given by Haberzettl is implemented in practice for photoproduction amplitudes at the tree level with form factors describing composite nucleons. We demonstrate that, in contrast to Ohta's gauge-invariance prescription, this formalism allows electric current contributions to be multiplied by a form factor, i.e., it does not require that they be treated like bare currents. While different in detail, this nevertheless lends support to previous ad hoc approaches which multiply the Born amplitudes by an overall form factor. Numerical results for kaon photoproduction off the nucleon are given. They show that the gauge procedure by Haberzettl leads to much improved $\chi^2$ values as compared to Ohta's prescription., Comment: 5 pages, RevTeX, two eps figures