Search

The reactions π−p → ηn and γp → ηp are investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region. The meson-baryon channels included are πN, ηN, πΔ, σN, and σN. The direct η-photoproduction process is studied within a formalism based on a chiral constituent quark model approach, complemented with a one-gluon-exchange mechanism, to take into account the breakdown of the SU (6) ⊗ O (3) symmetry. In the models search, the following known nucleon resonances are embodied: S11(1535), S11(1650), P11(1440), P11(1710), P13(1720), D13(1520), D13(1700), D15(1675), and F15(1680). Data for the π–p → ηn reaction from threshold up to a total center-of-mass energy of W ≈ 2 GeV are satisfactorily reproduced. For the photoproduction channel, two additional higher mass known resonances, P13(1900) and F15(2000), are also considered. However, reproducing the data for γp → ηp requires, within our approach, two new nucleon resonances, for which we extract mass and width.

A constituent quark model is developed for the reaction, allowing us to investigate all available data for differential cross sections as well as single polarization asymmetries (beam and target) by including {\it all} of the PDG, one to four star, nucleon resonances ($S_{11}$, $P_{11}$, $P_{13}$, $D_{13}$, $D_{15}$, $F_{15}$, $F_{17}$, $G_{17}$, $G_{19}$, $H_{19}$, $I_{1,11}$, and $K_{1,13}$). Issues related to the missing resonances are also briefly discussed by examining possible contributions from several new resonances ($S_{11}$, $P_{11}$, $P_{13}$, $D_{13}$, $ D_{15}$, and $H_{1,11}$).

Results for the reaction γ p → K + Λ , studied within a constituent quark model and a dynamical coupled-channel approach, are presented and compared with recent data. Issues related to the search for missing baryon resonances are briefly discussed and the role played by a third S 11 resonance is underlined.

Within a quark model the process γp → ηp , for E γ lab D 13 (1520), F 15 (1680), and to a lesser extent from P 13 (1720) in addition to those from the dominant S 11 (1535) resonance, implied by the Mainz differential cross-section data. The contributions from the resonances S 11 (1650), D 13 (1700) and D 15 (1675) are found consistently small but sensitive to the polarized target asymmetry measured recently at ELSA. These results are shown to be consistent with the quark model predictions.

A brief review of the elementary reactions in the kaons electroproduction associated with the A or Σ hyperons is presented. Some developments in recent theoretical approaches are emphasized with respect to the reaction mechanism, off shell effects, and the electromagnetic form factors of kaons and kaonic resonances.

Previous approaches to the photo- and electro-production of strangeness off the proton, based upon effective hadronic Lagrangians, are extended here to incorporate the so called off-shell effects inherent to the fermions with spin >= 3/2. A formalism for intermediate-state, spin 3/2, nucleonic and hyperonic resonances is presented and applied to the processes $\gamma + p ---> K^{+} + \Lambda$, for $E_{\gamma}^{lab}$ e' + K^+ + \Lambda$, as well as the branching ratio for the crossed channel reaction $K^- + p ---> \gamma + \Lambda$, with stopped kaons. The sensitivity, from moderate to significant, of various observables to such effects are discussed., Comment: 38 pages in LaTeX2e-RevTeX + 8 postscript figures

We present new break-up and coherent data for $\ensuremath{\eta}$ meson photoproduction on the deuteron, using a deuterium target and tagged bremsstrahlung photons up to 1 GeV. The differential cross sections for the coherent process were measured from threshold to 800 MeV. They are much smaller than those previously reported. The break-up channel provides a direct measurement of the neutron to proton differential cross section ratios. At the ${S}_{11}(1535)$ resonance peak, ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.68\ifmmode\pm\else\textpm\fi{}0.06$ leading to an isoscalar to isovector amplitude ratio of ${A}_{s}{/A}_{\ensuremath{\upsilon}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.096\ifmmode\pm\else\textpm\fi{}0.02$.

Our earlier Faddeev three-body study in the $K^-$-deuteron scattering length, $A_{K^-d}$, is revisited here in the light of the recent developments in two fronts: {\it (i)} the improved chiral unitary approach to the theoretical description of the coupled $\Kbar N$ related channels at low energies, and {\it (ii)} the new and improved measurement from SIDDHARTA Collaboration of the strong interaction energy shift and width in the lowest $K^-$-hydrogen atomic level. Those two, in combination, have allowed us to produced a reliable two-body input to the three-body calculation. All available low-energy $K^-p$ observables are well reproduced and predictions for the $\Kbar N$ scattering lengths and amplitudes, $(\pi \Sigma)^\circ$ invariant-mass spectra, as well as for $A_{K^-d}$ are put forward and compared with results from other sources. The findings of the present work are expected to be useful in interpreting the forthcoming data from CLAS, HADES, LEPS and SIDDHARTA Collaborations., Comment: 34 pages, 5 figures, text shortened, references updated, figures improved, typos fixed, version accepted for Phys. Rev. C

Total cross sections, differential cross sections and asymmetries for the reaction p( p ,π 0 )pp from threshold to 1 GeV have been measured. The outgoing π0 was detected by a 4π neutral meson spectrometer. Our data are compared with the corresponding np→NNπ± reactions. The isoscalar partial cross sections are extracted from the relevant total cross sections. They are significant throughout the whole energy range. Below the Δ-region, energy-integrated differential cross sections show a large anisotropy difference between the neutral and charged pion channels, respectively. A partial-wave fit shows that the initial T = 0 state is dominated by the 3D1 wave in this energy region. Finally, predictions of phenomenological models give reasonable good account of the measured observables but cannot reproduce the sign of the asymmetries.

An extended chiral constituent quark approach, which embodies higher Fock five-quark components in the baryons wave-functions, is employed to study the sea quark content of the octet baryons. We report on the probabilities of $\bar{u}$, $\bar{d}$ and $\bar{s}$ in the nucleon, $\Lambda$, $\Sigma$ and $\Xi$ baryons, arising from the intrinsic five-quark states in the baryons wave functions. Based on those probabilities, results for meson-nucleon $\sigma$-terms; namely $\sigma_{\pi N}, \sigma_{K N}$ and $\sigma_{\eta N}$, are presented.

Partial decay widths of the nucleon resonances S11(1535), S11(1650), D13(1520) and D13(1700) to the pseudoscalar mesons and octet baryons are studied within a chiral constituent quark model. The effects of the configurations mixing between the states |N 2 8 PMiand |N 4 8 PMiare considered, taking into account the breakdown of the SU(6) ⊗ O(3) symmetry. In addition, possible contributions of the strangeness components in the S11 resonances are investigated. Experimental data for the partial decay widths of the S11 and D13 resonances are well reproduced. Predictions for coupling constants of the four nucleon resonances to pseudoscalar mesons and octet baryons, crucial issues in the photo- and hadron-induced meson production reactions, are reported.

Coupling constants of $S_{11}(1535)$ and $S_{11}(1650)$ resonances to meson-baryon ($\pi N$, $\eta N$, $\eta^\prime N$, $KY$) are investigated in a chiral quark model, complemented by inclusion of five-quark components in those resonances. The known strong decay partial widths of both resonances are well reproduced, provided sizeable strangeness components in the relevant wave functions. %to excellent description for the strong decays of the above two resonances, which %cannot be reproduced very well in the traditional chiral quark model with or without %$SU(6)\otimes O(3)$ breaking. Finally, we find that $S_{11}(1535)$ and $S_{11}(1650)$ %couple strongly to the channels with strangeness., Comment: typo corrected

The NN\ensuremath{\rightarrow}N\ensuremath{\Delta} reaction is investigated within the coupled \ensuremath{\pi}NN-NN theory. The model predictions for the differential cross section, production asymmetry, and spin-correlation parameters are compared with the Argonne data at ${\mathit{T}}_{\mathit{N}}^{\mathrm{lab}}$=0.57, 0.81, and 1.01 GeV. Short-range effects are introduced through the \ensuremath{\rho}-exchange contribution. The overall quality of the results is satisfactory, except for the triplet part of the production asymmetry at the two highest energies. The role of the $^{3}$${\mathit{F}}_{3}$${\ensuremath{\rightarrow}}^{5}$${\mathit{P}}_{3}$ amplitude in this problem is assessed.

A chiral constituent quark model approach, embodying s- and u-channel exchanges,complemented with a Reggeized treatment for t-channel is presented. A model is obtained allowing data for $\pi^- p \to \eta n$ and $\gamma p \to \eta p$ to be describe satisfactorily. For the latter reaction, recently released data by CLAS and CBELSA/TAPS Collaborations in the system total energy range $1.6 \lesssim W \lesssim 2.8$ GeV are well reproduced due to the inclusion of Reggeized trajectories instead of simple $\rho$ and $\omega$ poles. Contribution from "missing" resonances is found to be negligible in the considered processes., Comment: 23 pages.4 figures,4 tables, to appear in Phys.Rev.C

Within an extended chiral constituent quark model, the three- and five-quark structure of the S-01 resonance Lambda(1405) is investigated. Helicity amplitudes for electromagnetic decays [Lambda(1405)->Lambda(1116)gamma, Sigma(1194)gamma] and transition amplitudes for strong decays [Lambda(1405)->Sigma(1194)pi, K- p] are derived, as well as the relevant decay widths. The experimental value for the strong decay width, Gamma(Lambda(1405)->(Sigma pi)degrees) = 50 +/- 2MeV, is well reproduced with about 50% of a five-quark admixture in the Lambda(1405). Important effects owing to the configuration mixing among Lambda P-2(1)A, Lambda P-2(8)M, and Lambda P-4(8)M are found. In addition, transitions between the three- and the five-quark components in the baryons turn out to be significant in both radiative and strong decays of the Lambda(1405) resonance.

Observables in two-body reactions such as cross sections, polarizations, and spin correlations are often of simple form at threshold, since only a few partial waves contribute. Spin observables near threshold for meson photoproduction reactions (with emphasis on the \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathit{K}}^{+}$\ensuremath{\Lambda} case) are studied in this paper. The expected nodal structure of all sixteen observables near threshold, their evolution with momentum, the role of the centrifugal barrier on threshold multipole amplitudes, and the role of resonance effects on spin observables are discussed.

The \ensuremath{\pi}NN system is studied with two-term potentials for the \ensuremath{\pi}N interaction in the ${\mathit{P}}_{33}$ and ${\mathit{P}}_{11}$ channels. Such model systematics as the difference in the dominant \ensuremath{\pi}N ${\mathit{P}}_{33}$ input, the effect of the so-called backward propagating pion, the choice of the relative momenta (magic vectors) at the isobar vertices, etc., are investigated. The results in the \ensuremath{\pi}d-\ensuremath{\pi}d, \ensuremath{\pi}d-NN, and NN-NN reactions at intermediate energies are compared with the available experimental data. The overall quality of the predictions by the present approach is roughly comparable to that based upon the one-term ${\mathit{P}}_{33}$ isobar model supplemented by an appropriate ad hoc off-shell modification, but with a significant improvement in the description of the \ensuremath{\pi}d-NN asymmetry ${\mathit{A}}_{\mathit{y}0}$.

Within a chiral constituent quark model approach, {eta}-meson production on the proton via electromagnetic and hadron probes is studied. With few parameters, the differential cross section and polarized beam asymmetry for {gamma}p{yields}{eta}p and differential cross section for {pi}{sup -}p{yields}{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{sub 11}(1535), S{sub 11}(1650), P{sub 13}(1720), D{sub 13}(1520), and F{sub 15}(1680) are found to be dominant in the reaction mechanisms in both channels. Possible roles played by new resonances are also investigated; and in the photoproduction channel, significant contribution from S{sub 11} and D{sub 15} resonances, with masses around 1715 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*{yields}{pi}N, {eta}N are also presented and found to be consistent with the Particle Data Group values.

The reaction ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\eta}n$ is investigated within a dynamical coupled-channels model of meson production reactions in the nucleon resonance region. The meson baryon channels included are $\ensuremath{\pi}N,\ensuremath{\eta}N,\ensuremath{\pi}\ensuremath{\Delta},\ensuremath{\sigma}N$, and $\ensuremath{\rho}N$. The nonresonant meson-baryon interactions of the model are derived from a set of Lagrangians by using a unitary transformation method. One or two excited nucleon states in each of $S,P,D$, and $F$ partial waves are included to generate the resonant amplitudes. Data of the ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\eta}n$ reaction from threshold up to a total center-of-mass energy of about 2 GeV are satisfactorily reproduced and the roles played by the following nine nucleon resonances are investigated: ${S}_{11}(1535),{S}_{11}(1650),{P}_{11}(1440),{P}_{11}(1710),{P}_{13}(1720),{D}_{13}(1520),{D}_{13}(1700),{D}_{15}(1675)$, and ${F}_{15}(1680)$. The reaction mechanism and the predicted $\ensuremath{\eta}N$ scattering length are discussed.

The process {gamma}{ital p}{r arrow}{ital K}{sup +}{Lambda} is studied in a phenomenological approach. A thorough analysis of the existing differential cross section data up to 1.4 GeV, taking into account the systematic uncertainties, shows the Orsay data to be internally inconsistent. Removing this data set from the data base results in a rather simple reaction mechanism with the extracted main kaon-hyperon-nucleon coupling constants in agreement with the SU(3) predictions. Employing a diagrammatic model, the differential and total cross section, {Lambda}-polarization, and polarized target data are well reproduced. Predictions for other single and double polarization observables and suggestions for new measurements are presented. Applying crossing symmetry, the reaction is related to the radiative {ital K}{sup {minus}} capture process, and a comparison with the measured branching ratio is made.

A chiral quark-model approach is extended to the study of the $\ensuremath{\pi}N$ scattering at low energies. The process of ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}\ensuremath{\eta}n$ near threshold is investigated. The model is successful in describing the differential cross sections and total cross section near the \ensuremath{\eta} production threshold. The roles of the resonances in $n\ensuremath{\leqslant}2$ shells are clarified. Near threshold, the ${S}_{11}(1535)$ dominates the reactions, whereas the interferences from the ${S}_{11}(1650)$ turn out to be destructive around $W\ensuremath{\lesssim}1.6$ GeV. The ${D}_{13}(1520)$ is crucial to give correct shapes of the differential cross sections. The nucleon pole term contributions are significant. The ${P}_{11}(1710)$ plays an important role around the center-of-mass energy $W=1.7$ GeV, it is crucial to produce an enhancement in the region of $Wg1.6$ GeV as suggested by the data for total cross section. The $t$-channel is negligible in the reactions.

A dynamical coupled-channels formalism for processes $\ensuremath{\pi}N\ensuremath{\rightarrow}\mathit{KY}$ and $\ensuremath{\gamma}N\ensuremath{\rightarrow}\mathit{KY}$ is presented that provides a comprehensive investigation of recent data on the $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}$ reaction. The nonresonant interactions within the subspace $\mathit{KY}\ensuremath{\bigoplus}\ensuremath{\pi}N$ are derived from effective Lagrangians, using a unitary transformation method. The calculations of photoproduction amplitudes are simplified by casting the coupled-channels equations into a form such that the empirical $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}N$ amplitudes are input and only the parameters associated with the $\mathit{KY}$ channel are determined by performing ${\ensuremath{\chi}}^{2}$ fits to all of the available data for ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{K}^{\ifmmode^\circ\else\textdegree\fi{}}\ensuremath{\Lambda},{K}^{\ifmmode^\circ\else\textdegree\fi{}}{\ensuremath{\Sigma}}^{\ifmmode^\circ\else\textdegree\fi{}}$, and $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}$. Good agreement between our models and those data are obtained. In the fits to $\ensuremath{\pi}N\ensuremath{\rightarrow}\mathit{KY}$ channels, most of the parameters are constrained within $\ifmmode\pm\else\textpm\fi{}20%$ of the values given by the Particle Data Group and/or quark model predictions, whereas for $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}$ parameters, ranges compatible with broken SU(6) \ensuremath{\bigotimes} O(3) symmetry are imposed. The main reaction mechanisms in ${K}^{+}\ensuremath{\Lambda}$ photoproduction are singled out and issues related to newly suggested resonances ${S}_{11},{P}_{13}$, and ${D}_{13}$ are studied. Results illustrating the importance of using a coupled-channels treatment are reported. Meson cloud effects on the $\ensuremath{\gamma}N\ensuremath{\rightarrow}{N}^{*}$ transitions are also discussed.

Differential cross-section and single polarization observables in the process gamma p --> K^+ Lambda are investigated within a constituent quark model and a dynamical coupled-channel formalism. The effects of two new nucleon resonances and of the K*(892)- and K1(1270)-exchanges are briefly presented., Contributed paper to the IVth International Conference on Quarks and Nuclear Physics, Madrid June 5-10, 2006

The pi N --> KY and KY --> KY reactions are studied using a dynamical coupled-channel model of meson-baryon interactions at energies where the baryon resonances are strongly excited. The channels included are: pi N, K \Lambda, and K\Sigma. The resonances considered are: N^* [S_{11}(1650), P_{11}(1710), P_{13}(1720),D_{13}(1700)]; \Delta^* [S_{31}(1900), P_{31}(1910), P_{33}(1920)]; \Lambda ^* [S_{01}(1670), P_{01}(1810)] \Sigma^* [P_{11}(1660), D_{13}(1670)]; and K^*(892). The basic non-resonant \pi N --> KY and KY --> KY transition potentials are derived from effective Lagrangians using a unitary transformation method. The dynamical coupled-channel equations are simplified by parametrizing the pi N -->pi N amplitudes in terms of empirical pi N partial-wave amplitudes and a phenomenological off-shell function. Two models have been constructed. Model A is built by fixing all coupling constants and resonance parameters using SU(3) symmetry, the Particle Data Group values, and results from a constituent quark model. Model B is obtained by allowing most of the parameters to vary around the values of model A in fitting the data. Good fits to the available data for pi^- p to K^0 \Lambda, K^0 \Sigma^0 have been achieved. The investigated kinematics region in the center-of-mass frame goes from threshold to 2.5 GeV. The constructed models can be imbedded into associated dynamical coupled-channel studies of kaon photo- and electro-production reactions., Comment: 35 pages, 11 Figures

$\Kbar N$ interactions are investigated {\it via} an effective non-linear chiral meson-baryon Lagrangian. The adjustable parameters are determined by a fitting procedure on the $K^-p$ threshold branching ratios and total cross-section data for $p^{lab}_K\le$ 250 MeV/c. We produce predictions for the $\Sigma \pi$ mass spectrum, and scattering lenghts $a_{K^-p}$, $a_n(K^-n \to K^-n)$, $a_n0(\Kbar0 n \to \Kbar0 n)$, and $a_{ex}(K^-p \to \Kbar0 n)$. The $\Kbar N$ amplitudes thus obtained, as well as those for other two-body channels ($\pi N$, $NN$, and $YN$) are used as input to predict the scattering length $A_{K^-d}$, for which we have devised a relativistic version of the three-body Faddeev equations. Results for all two- and three-body coupled channels are reported both in isospin and particle bases. All available $\Kbar N$ data are well reproduced and our best results for the $K^-p$ and $K^-d$ scattering lenghts are $a_{K^-p} = (-0.90 + i 0.87) fm$ and $A_{K^-d} = (-1.80 + i 1.55) fm$.

A brief review of the associated strangeness electromagnetic production is presented. Very recent K+ Lambda photoproduction data on the proton from threshold up to E=2.6 GeV are interpreted within a chiral constituent quark formalism, which embodies all known nucleonic and hyperonic resonances. The preliminary results of this work are reported here., 12 pages, 4 figures; Invited talk given at the International Symposium on Electrophoto-production of Strangeness on Nucleons and Nuclei, Sendai, Japan, June 16-18, 2003; World Scientific (to appear)

Electromagnetic meson production formalisms are reviewed, with emphasise placed on their ability in search for new baryon resonances via gamma p -> K+ Lambda and gamma p -> eta p processes. The relevant studies, aiming to deepen our insights to hadron spectroscopy, constitute strong tests of the QCD inspired theoretical developements., 10 pages, Invited talk given at "International Symposium on Hadrons and Nuclei", Seoul, February 20 - 22, 2001

Within a chiral constituent quark formalism, allowing the inclusion of all known resonances, a comprehensive study of the recent eta photoproduction data on the proton up to 2 GeV is performed. This study shows evidence for a new S11 resonances and indicates the presence of an additional missing P13 resonance., 10 pages, 3 figures, Invited talk given at the International Symposium on Electromagnetic Interactions in Nuclear and Hadron Physics (EMI2001), Osaka, Japan, 4-7 Dec. 2001

A recently developed quark model approach to pseudoscalar meson photoproduction is extended to electroproduction process for the $\eta$ meson in the kinematics of momentum transfer $Q^2 \leq$ 4 (GeV/c)$^2$ and total center of mass energy $W \leq$ 1.6 GeV. Existing data are well reproduced and the roles of the $S_{11}(1535)$ and $D_{13}(1520)$ resonances are closely investigated. In the study of the longitudinal excitation of the $S_{11}(1535)$ resonance, a reliable constraint on the $S_{11}(1535)$ properties is obtained by cleanly removing the electromagnetic transition from the $\gamma_{(v)} p \to S_{11}(1535) \to \eta p$ amplitude. Thus, the fitted quantities can be determined with an uncertainty of about 15%. This could be the first direct constraint on the $S_{11}(1535)$ properties in theory., Comment: 31 pages, 9 figures; Substantially modified version, to appear on Jour. of Phys. G

The process $\gamma p \to \phi p$ close to threshold is investigated focusing on the role played by the {\it s}- and {\it u}-channel nucleonic resonances. For this purpose, a recent quark model approach, based on the $SU(6)\otimes O(3)$ symmetry with an effective Lagrangian, is extended to the $\phi$ meson photoproduction. Another non-diffractive process, the {\it t}-channel $\pi^0$ exchange, is also included. The diffractive contribution is produced by the {\it t}-channel Pomeron exchange. Contributions from non-diffractive {\it s}- and {\it u}-channel process are found small in the case of cross sections and polarization observables at forward angles. However, backward angle polarization asymmetries show high sensitivity to this non-diffractive process. Different prescriptions to keep gauge invariance for the Pomeron exchange amplitudes are investigated. Possible deviations from the exact $SU(6)\otimes O(3)$ symmetry, due to the configuration mixing, are also discussed., Comment: 34 pages, 16 figures, to appear in Nucl. Phys. A

Results of recent theoretical studies on the electromagnetic form factors of strange baryons and kaons are embodied in a strangeness electromagnetic production model based on an effective Lagrangian approach. The sensitivity of different observables that are the subject of current or planned measurements at JLAB, are investigated with respect to the phenomenological ingredients of the model.

Previous approaches to the photo- and electro-production of strangeness on the proton, based upon effective Lagrangian, is extended to incorporate the so called off-shell effects (OSE) required while dealing with spin ⩾3/2 baryonic resonances. Results for K+ Λ channels are presented.

The general nodal structure approach is applied to the recent γp→ηp T-asymmetry data from ELSA. The reaction mechanism is found to require, in addition to the dominant S11 and D13 resonances, contributions from P13 and D15 resonances. This finding is confirmed within a simple dynamical approach. An indication on the presence of a predicted P13 nucleonic resonance is observed.

A formalism, based on an isobaric approach using Feynman diagrammatic techniques, which includes the nucleonic (spin \ensuremath{\le} 5/2), hyperonic (spin 1/2), and kaonic resonances, is developed. Using this formalism, a thorough investigation of the following electromagnetic strangeness processes, for which experimental results are available, is performed: \ensuremath{\gamma}p\ensuremath{\rightarrow}${\mathit{K}}^{+}$\ensuremath{\Lambda}, ${\mathit{K}}^{+}$${\mathrm{\ensuremath{\Sigma}}}^{0}$, ${\mathit{K}}^{0}$${\mathrm{\ensuremath{\Sigma}}}^{+}$, for ${\mathit{E}}_{\ensuremath{\gamma}}^{\mathrm{lab}}$\ensuremath{\le} 2.1 GeV, ep\ensuremath{\rightarrow}${\mathit{e}}^{\ensuremath{'}}$${\mathit{K}}^{+}$\ensuremath{\Lambda}, ${\mathit{e}}^{\ensuremath{'}}$${\mathit{K}}^{+}$${\mathrm{\ensuremath{\Sigma}}}^{0}$, and ${\mathit{K}}^{\mathrm{\ensuremath{-}}}$p\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\Lambda},\ensuremath{\gamma}${\mathrm{\ensuremath{\Sigma}}}^{0}$. A reaction mechanism, describing well enough the data, is found to include a reasonable number of baryonic resonances among a very large number of potential candidates. The extracted main kaon-hyperon-nucleon coupling constants are in good agreement with values predicted using SU(3) symmetry. The main findings of this model are compared with the results of other recent phenomenological studies. Predictions for the upcoming photoproduction polarization and electroproduction observables are presented, and their sensitivity to the phenomenological models ingredients are emphasized. \textcopyright{} 1996 The American Physical Society.

Spin observables for the reaction γp→K+Λ are examined using three recent dynamical models and are compared to the general features of such observables deduced recently by Fasano, Tabakin, and Saghai. An instructive surprise, which occur in this comparison, is then discussed showing a clear indication of the duality hypothesis in the strangeness realm.

Starting from an appropriate low energy model, we investigate the role played by spin‐3/2 nucleonic resonances. A new model is obtained giving good agreement with data up to 2.1 GeV and satisfying SU(3) constraints on the main coupling constants.

Spin observables for the reaction γp→ηp are examined using an isobaric dynamical model and fitting the recent data from Bonn. Studying possible contributions of ten isospin 1/2 nucleonic resonances, an excellent fit is obtained by a model including only three of them: S11 (1535), D13 (1520), and F15 (1680). No evidence for a significant contribution from the Roper resonance is found. Predictions for polarization observables are shown and compare well with the very scarce and old data. General rules for such observables deduced recently by Fasano, Tabakin, and Saghai are satisfied by the reported model.

A formalism, based on an isobaric approach, via Feynman diagrammatic technique, including the nucleonic (spin ≤ 5/2), hyperonic (spin 1/2) and kaonic resonances is developed. Using this formalism, a thorough investigation of all the electromagnetic strangeness processes, for which experimental results are available, is performed and satisfactory agreement with the data is obtained. The reaction mechanism is found to include a reasonable number of baryonic resonances, with the extracted main kaon-hyperon-nucleon coupling constants in agreement with the SU(3)-symmetry predictions. The sensitivity of this model to the form factors of the relevant hadrons is emphasized.

We have measured total and differential cross sections for η‐meson photo‐production on 7 cm thick 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV, using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The photoreaction was identified by detecting the η decay products in the neutral meson spectrometer SPES0‐2π, while the recoil baryons (proton, neutron, or deuteron) were detected by a variety of large angle scintillator detectors. Some of our recent preliminary results, both theoretical and experimental, will be discussed.

Investigations in progress on the associated strangeness photo‐ and electroproduction of the proton in the few GeV region and the negative kaon radiative capture are reported, and results for the K+∑0 channel are presented.

Spin observables for the reaction $\gamma p \rightarrow K^+ \Lambda$ are examined using three recent dynamical models and are compared to the general features of such observables deduced earlier by Fasano, Tabakin and Saghai. These general features, such as the energy dependence of spin observables and the location of nodes in their angle dependence, are realized. Several instructive surprises, which occur in this comparison to the conjectures of Fasano et al., are then discussed. The sensitivity of spin observables to isobar and {\it t-}channel dynamics is analyzed and suggestions for selecting experiments which provide important dynamical information are presented., Comment: 22 pages, latex, 10 figures

The associated strangeness production in pp collisions was studied at bombarding energies of 2.3 and 2.7 GeV by detecting the outgoing K + particles with a high resolution magnetic spectrometer at forward angles up to 23.5° laboratory angle. The kaons were separated from an immense background of protons and pions by a highly resolving TOF electronics addition to vetoes from Cerenkov detectors utilizing either β differing Cerenkov light cones in lucite or different thresholds for light production in aerogel. The hyperon-nucleon missing mass spectra obtained show strong deviations from pure phase space which is ascribed to final state interactions of the pΛ and the Nσ systems. The structures found are discussed and compared with results from K − d → π − YN experiments, with theoretical calculations in the frame of OBE models, and with the prediction of sharp strange dibaryon resonances from several quark-bag models.

The cross section for exclusive ${\mathrm{\ensuremath{\pi}}}^{+}$ electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (\ensuremath{\varepsilon}\ensuremath{\sim}0.2 and \ensuremath{\varepsilon}\ensuremath{\sim}0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors ${\mathit{G}}_{\mathit{A}}$ and ${\mathit{G}}_{\mathit{P}}$ at \ensuremath{\Vert}t\ensuremath{\Vert}=0.073, 0.139, and 0.179 (GeV/c${)}^{2}$. The slope of ${\mathit{G}}_{\mathit{A}}$ agrees with the value obtained in neutrino experiments. ${\mathit{G}}_{\mathit{P}}$ satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.

A direct experimental determination has been made of the ratio of the forward-angle positive-pion electroproduction cross section for a proton bound in the deuteron with that of a free proton for invariant masses of 1160 and 1232 MeV. A significant quenching of the reaction in the deuteron is observed.

A strong enhancement at the (Σ Open image in new window )+ threshold is observed in missing mass spectra of the reaction pp→K+X experiment performed at the French Saturn National Lab (LNS). This effect is related to the coupling of the (Σ Open image in new window )+ and Λp channels. It could be the signature of a strangeS=−1 resonance the H1* (2130),SU(3) analog partner of the deuteron.

There has been a long-standing problem in the coupled \ensuremath{\pi}NN-NN theory concerning its apparent failure in reproducing the tensor polarization ${T}_{20}$ in the elastic \ensuremath{\pi}d scattering at medium energies. A mechanism recently proposed by Jennings to solve this problem is examined. It is then found that it comes from the positive-energy pole in one of the nucleon propagators in the exchange impulse diagram within the covariant picture which is usually overlooked when devising the spectator on-mass-shell approximation, and that it does not lead to the claimed near cancellation of the Pauli exchange contribution, but is important to be included. The inclusion of this contribution is shown to considerably improve in reproducing the pion-deuteron spin observables including ${T}_{20}$, particularly above the delta resonance energy.

The coupled πNN-NN theory is examined in its dominant πN P33 (or Δ) input by a direct calculation of the NN → NΔ observables which are then compared with the recent data on pp → Δ+ + n extracted from the pp → π+ pn reaction at medium energies.

A new interest to study the S = -1, B = 2 system has recently been raised due to theoretical predictions coming either from both potential [1] and quark bag model calculations [2] about the existence of narrow exotic states. In particular the NIJMEGEN GROUP [j] predicts in the MIT bag model a large number of 6 quark configurations with strangeness S = 0, -1 or -2. The stability of these states against quark dissociation or two baryon emission increases with the number of strange quarks involved, the optimum case beeing for the S = -2 system (uu dd ss) that could even present a stable di-hyperon state [4]. No experimental evidence for the observation of such S = -2 states has yet been found, but the cross section level reached in these different experimental researches [5] does not rule out their existence.