arXiv subjects
Publications and source records attributed to C. Bennhold.
The electromagnetic form factors of strange mesons and baryons are studied by means of kaon electroproduction on the nucleon. The response functions that are sensitive to the K0, Lambda, Sigma, and KK*γtransition form factors are systematically explored. The effects of these form factors on several response functions are discussed.
A quark model approach to the photoproduction of vector mesons off nucleons is proposed. Its starting point is an effective Lagrangian of the interaction between the vector meson and the quarks inside the baryon. This paper develops the formalism and highlights the dynamical roles of s-channel resonances. It is demonstrated that the proposed approach is ideal to support searching for the missing resonances in the vector meson photoproduction experiments proposed at TJNAF.
The electroproduction of kaons on the nucleon is shown to be an ideal tool to extract electromagnetic form factors of strange mesons and hyperons. Longitudinal n(e,e'K0)Lambda cross sections are found to be sensitive to the K0 form factor, while sensitivity to the Lambda magnetic form factor can be seen in recoil-beam double polarization observables. The KK*γtransition form factor could be extracted by measuring transverse response functions in the K-Sigma channels.
In the framework of the impulse approximation we study the photoproduction of the hypertriton $^3_Λ$H by using realistic $^3$He wave functions obtained as solutions of Faddeev equations with the Reid soft-core potential for different $^3_Λ$H wave functions. We obtain relatively small cross sections of the order of 1 nb. We also find that the influence of Fermi motion is important, while the effect of different off-shell assumptions on the cross section is not too significant.
Kaon photo- and electroproduction off the nucleon is investigated in the framework of an isobaric model up to energies of about 1 GeV above threshold for all six isospin channels. The hadronic coupling constants in the model are determined by fitting to the available experimental data. We give a brief status report on the state of these models and discuss problems to be attacked in the future. We include hadronic form factors and show that they are essential for a proper description of the dynamics even though their inclusion has to be done carefully in order to preserve gauge invariance. We also investigate the effects of the K0 electromagnetic form factors for two quark models on the longitudinal and transverse differential cross sections. It is found that only the reaction n(e,e'K0)Λis sufficiently sensitive to extract the K0 form factor.
The nonmesonic decay of the hypertriton is calculated based on a hypertriton wavefunction and 3N scattering states, which are rigorous solutions of 3-body Faddeev equations using realistic NN and hyperon-nucleon interactions. The pion-exchange together with heavier meson exchanges for the $ΛN \to N N $ transition is considered. The total nonmesonic decay rate is found to be 0.5% of the free $Λ$ decay rate. Integrated as well as differential decay rates are given. The p- and n- induced decays are discussed thoroughly and it is shown that the corresponding total rates cannot be measured individually.
The nonmesonic weak decay of $Λ$ hypernuclei is studied in a shell model framework. A complete strangeness-changing weak $ΛN \to NN$ transition potential, based on one boson exchange, 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)$_w$. General expressions for nucleons in arbitrary shells are obtained. The transition matrix elements include realistic $Λ$N short-range correlations and NN final state interactions based on the Nijmegen baryon-baryon potential. The decay rates are found to be especially sensitive to the inclusion of the strange mesons, K and K$^*$, even though the role of kaon exchange is found to be reduced with recent couplings obtained from next-to-leading order Chiral Perturbation Theory. With the weak couplings used in this study the rates remain dominated by the pion-exchange mechanism since the contributions of heavier mesons either cancel each other or are suppressed by form factors and short-range correlations. The total decay rate therefore remains in agreement with present measurements. However, the partial rates which are even more sensitive to the inclusion of heavier mesons cannot be reconciled with the data. The proton asymmetry changes by 50% once heavier mesons are included and agrees with the available data.
Kaon photo- and electroproduction in all six isospin channels are investigated by means of an isobaric model. It is found that existing models can only partially describe the current experimental data. Moreover, the extracted coupling constants are mostly inconsistent with the prediction of SU(3) symmetry or the information from hadronic reactions. In order to further analyze this problem, we introduce a form factor at the hadronic vertices $KNY$ (with $Y=Λ~ \textrm{or}~ Σ$) and study its behavior qualitatively. Our result suggests the need of more accurate experiments, especially in the charged $Σ$ and the neutral $K^0$ channels.
Quasifree $η$ photoproduction from nuclei is studied in the Distorted Wave Impulse Approximation (DWIA). The elementary eta production operator contains Born terms, vector meson and nucleon resonance contributions and provides an excellent description of the recent low energy Mainz measurements on the nucleon. The resonance sector includes the $S_{11}(1535)$, $P_{11}(1440)$ and $D_{13}(1520)$ states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for the $ηN$ t-matrix are used to construct a simple $ηA$ optical potential based on a $t ρ$-approximation. We find that the exclusive $A(γ,ηN)B$ process can be used to study medium modifications of the $N^*$ resonances, particularly if the photon asymmetry can be measured. The inclusive $A(γ, η)X$ reaction is compared to new data obtained on $^{12}C$, $^{40}Ca$, and is found to provide a clear distinction between different models for the $ηN$ t-matrix.
Pion elastic scattering on deuterium is studied in the KMT multiple scattering approach developed in momentum space. Using a Paris wave function and the same methods and approximations as commonly used in pion scattering on heavier nuclei excellent agreement with differential cross section data is obtained for a wide range of pion energies. Only for $T_π>250$ MeV and very backward angles, discrepancies appear that are reminiscent of disagreements in pion scattering on $^3$He, $^3$H, and $^4$He. At low energies the second order corrections have been included. Polarization observables are studied in detail. While tensor analyzing powers are well reproduced, vector analyzing powers exhibit dramatic discrepancies.
Coherent pion photoproduction, $d(γ,π^0)d$, is calculated in a coupled channels approach in momentum space using an elementary production operator that includes Born terms, vector meson exchange and $M1/E2$ $Δ$-resonance excitation. The final state interaction is treated in the KMT multiple scattering approach that describes $πd$ elastic scattering well. Differential cross sections are found to agree well with existing ($γ, π^0$) data. The polarization observables $Σ, iT_{11}$ and the tensor analyzing powers are predicted for forthcoming experiments. A number of transparent relationships between polarization observables and the elementary production operator is derived. Finally, calculations are performed for coherent eta photoproduction, $d (γ, η)d$, in the threshold region which agree well with a very recent Mainz experiment.
We present calculations of the reaction $A(e,e^\prime πN)B$ in the distorted wave impulse approximation. The reaction allows for the study of the production process in the nuclear medium without being obscured by the details of nuclear transition densities. First, a pion electroproduction operator suitable for nuclear calculations is obtained by extending the Blomqvist-Laget photoproduction operator to the virtual photon case. The operator is gauge invariant, unitary, reference frame independent, and describes the existing data reasonably well. Then it is applied in nuclei to predict nuclear cross sections under a variety of kinematic arrangements. Issues such as the effects of gauge-fixing, the interference of the $Δ$ resonance with the background, sensitivities to the quadrupole component of the $Δ$ excitation and to the electromagnetic form factors, the role of final-state interactions, are studied in detail. Methods on how to experimentally separate the various pieces in the coincidence cross section are suggested. Finally, the model is compared to a recent SLAC experiment.
The few available data for the reactions $γp \rightarrow K^{0} Σ^{+}$ and $γn \rightarrow K^{+} Σ^{-}$ are compared to models developed for the processes $γp \rightarrow K^{+} Σ^{0}$ and $γp \rightarrow K^{+} Λ$. It is found that some of these phenomenological models overpredict the measurements by up to a factor of 100. Fitting the data for all of these reactions leads to drastically reduced Born coupling constants.
A breakdown of the Impulse Approximation is studied in pion photoproduction on $^3$He at high momentum transfers. The usual DWIA formalism with Faddeev wave functions which works well for small momentum transfers deviates from experimental measurements by up to two orders of magnitude for $Q^2>6 fm^{-2}$. It is found that the explicit inclusion of two-body mechanisms, where the photon is absorbed on one nucleon and the pion is emitted from another nucleon can restore agreement with the data.
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 $S_{11}(1535)$, $P_{11}(1440)$ and $D_{13}(1520)$ 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.
Pion elastic, charge exchange scattering and induced eta production on the trinucleon systems are investigated in a coupled-channels approach in momentum space with Fadeev wave functions. The channel $πN \rightarrow ηN$ is included using an isobar model with S-, P-, and D-wave resonances. While the coherent reactions like $^3$He($π,π)^3$He can be reasonably well reproduced up to $T_π$=500 MeV, large discrepancies appear for the incoherent processes, $^3$He($π^-,π^0)^3$H and $^3$He($π^-,η)^3$H at backward angles and energies above $Δ$-resonance. In the forward direction the $(π,η)$ calculations underestimate the experimental measurements very close to threshold but agreement with the data improves with increasing pion energy. Predictions are made for the asymmetries of the various reactions on polarized $^3$He.