C. Bennhold

Low lying S = -1 excited baryons and chiral symmetry

Abstract The s-wave meson–baryon interaction in the S=−1 sector is studied by means of coupled-channels, using the lowest-order chiral Lagrangian and the N/D method to implement unitarity. The loops are regularized using dimensional renormalization. In addition to the previously studied Λ(1405), employing this chiral approach leads to the dynamical generation of two more s-wave hyperon resonances, the Λ(1670) and Σ(1620) states. We make comparisons with experimental data and look for poles in the complex plane obtaining the couplings of the resonances to the different final states. This allows us to identify the Λ(1405) and the Λ(1670) resonances with K N and KΞ quasibound states, respectively.

Measurement of γp→K+Λ and γp→K+Σ0 at photon energies up to 2 GeV

Abstract Associated strangeness production in the reactions γp → K + Λ and γp → K + Σ 0 was measured with the SAPHIR detector at the electron stretcher ring ELSA at Bonn. Data on total and differential cross sections and on hyperon polarizations are presented. The total cross section for Λ production shows a strong threshold enhancement whereas the Σ 0 data have a maximum at about E γ =1.45 GeV. Along with the angular decomposition of the differential cross section into polynomials, this suggests resonance production. However, the angular distributions of both hyperon polarizations vary only slightly with the photon energy. Λ and Σ 0 polarizations show opposite signs and change sign over the angular range.

The ηNN coupling in eta photoproduction

Abstract Low-energy eta photoproduction on the nucleon is studied in an effective lagrangian approach that contains Born terms, vector-meson and nucleon resonance contributions. The resonance sector includes the S11(1535), P11(1440) and D13(1520) states whose couplings are fixed by independent electromagnetic and hadronic data. The available (γ, η) data are employed to discuss the difference between pseudoscalar and pseudovector Born terms and to determine the magnitude of the ηNN coupling constant. We present multipoles that are most sensitive to the various model ingredients and demonstrate how these multipoles may be accessed in polarization observables. Cross-section calculations are presented for eta photoproduction on light nuclei.

Measurement of the reaction γp → K0Σat photon energies up to 2.6 GeV⋆

Abstract.The reaction γp → K0Σ+ was measured in the photon energy range from threshold up to 2.6 GeV with the SAPHIR detector at the electron stretcher facility, ELSA, in Bonn. Results are presented on the reaction cross-section and the polarization of the Σ+ as a function of the kaon production angle in the centre-of-mass system, cosΘc.m.K, and the photon energy. The cross-section is lower and varies less with photon energy and kaon production angle than that of γp → K+Σ0. The Σ+ is polarized predominantly at cosΘc.m.K ≈ 0. The data presented here are more precise than previous ones obtained with SAPHIR and extend the photon energy range to higher values. They are compared to isobar model calculations.

Weak decay of hypernuclei

The nonmesonic weak decay of light Λ hypernuclei is investigated in a shell model framework. The strangeness-changing weak ΛN → N N transition potential is constructed by including the exchange of the pseudoscalar mesons π, K, η as well as the vector mesons ρ, ω, and K*, whose weak coupling constants are obtained from soft meson theorems and SU(6)w. The transition matrix element includes a realistic ΛN short-range correlation function that is based on the Nijmegen Y N-potential. The total decay rate and the proton asymmetry, which represents a measure of the parity-violating to parity-conserving amplitudes, are in agreement with present experimental errors while the neutron- to proton-induced ratio is underestimated by more than a factor of two. In general, the observables are found to be dominated by the pion-exchange mechanism since the contributions of heavier mesons are suppressed by form factors and short-range correlations.

Alternative evaluations of halos in nuclei

Data for the scattering of {sup 6}He, {sup 8}He, {sup 9}Li, and {sup 11}Li from hydrogen are analyzed within a fully microscopic folding model of proton-nucleus scattering. Current data suggest that of these only {sup 11}Li has a noticeable halo. For {sup 6}He, we have also analyzed the complementary reaction {sup 6}Li({gamma},{pi}{sup +}){sup 6}He{sub g.s.}. The available data for that reaction support the hypothesis that {sup 6}He may not be a halo nucleus. However, those data are scarce and there is clearly a need for more to elicit the microscopic structure of {sup 6}He. (c) 2000 The American Physical Society.

Quasifree kaon photoproduction on nuclei

Abstract Investigations of the quasifree reaction A( γ , KY)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 multichannel 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.

Gauge-invariant tree-level photoproduction amplitudes with form factors

We show how the gauge-invariance formulation given by Haberzettl is implemented in practice for photoproduction amplitudes at tree level with form factors describing composite nucleons. We demonstrate that, in contrast to Ohta{close_quote}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 {ital 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}{sup 2} values compared to Ohta{close_quote}s prescription. thinsp {copyright} {ital 1998} {ital The American Physical Society}

The γ→p→K+Λ and γ→p→K+Σ0 reactions at forward angles with photon energies from 1.5 to 2.4 GeV

Differential cross sections and photon beam asymmetries for the gamma p rightarrow K+ Lambda and gamma p rightarrow K+ Sigma0 reactions have been measured in the photon energy range from 1.5 GeV to 2.4 GeV and in the angular range from Theta_{cm} = 0 to 60 of the K+ scattering angle in the center of mass system at the SPring-8/LEPS facility. The photon beam asymmetries for both the reactions have been found to be positive and to increase with the photon energy. The measured differential cross sections agree with the data measured by the CLAS collaboration at cosTheta_{cm} 0.9. In the K+Lambda reaction, the resonance-like structure found in the CLAS and SAPHIR data at W=1.96 GeV is confirmed. The differential cross sections at forward angles suggest a strong K-exchange contribution in the t-channel for the K+Lambda reaction, but not for the K+Sigma0 reaction.

Spin, parity, and nature of the xi(1620) resonance.

Using a unitary extension of chiral perturbation theory with a lowest-order s-wave SU(3) chiral Lagrangian we study low-energy meson-baryon scattering in the strangeness S=-2 sector. A scattering-matrix pole is found around 1605 MeV which corresponds to an s-wave Xi resonance with J(P)=1/2(-). We identify this resonance with the Xi(1620) state, quoted by the Particle Data Group with I=1/2 but with unknown spin and parity. The addition of the S=-2 state to the recently computed Lambda(1670), Sigma(1620), and N(1535) states completes the octet of J(P)=1/2(-) resonances dynamically generated in this chiral unitary approach.