Search

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

The effects of the initial state interactions on the {ital K}{sup {minus}}-{ital p} radiative capture branching ratios are examined and found to be quite sizable. A general coupled-channel formalism for both strong and electromagnetic channels using a particle basis is presented, and applied to all the low energy {ital K}{sup {minus}}-{ital p} data with the exception of the 1{ital s} atomic level shift. Satisfactory fits are obtained using vertex coupling constants for the electromagnetic channels that are close to their expected SU(3) values.

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.

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.

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.

$\Kbar N$ 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 $A_{K^-d} = (-1.80 + i 1.55) fm$.

We report on the first calculation of the scattering length ${A}_{{K}^{\ensuremath{-}}d}$ based on a relativistic three-body approach where the $\overline{K}N$ coupled channel two-body input amplitudes have been obtained with the chiral $\mathrm{SU}(3)$ constraint, but with isospin symmetry breaking effects taken into account. Results are compared with a recent calculation applying a similar set of two-body amplitudes, based on the fixed center approximation, and for which we find significant deviations from the three-body results. Effects of the deuteron D-wave component, pion-nucleon, and hyperon-nucleon interactions are also evaluated.

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 effect of the coupling to the ..pi..d channel on the intermediate energy NN scattering is studied using the recent NN..-->..NN and NN..--> pi..d partial wave analyses as well as the coupled NN..--> pi..NN theory. Contrary to some recent claims, it is found that this coupling is important in the /sup 1/D/sub 2/ and /sup 3/F/sub 3/ partial waves in which the intermediate N..delta.. configuration dominates.