arXiv subjects
Publications and source records attributed to C. Bennhold.
We present a nonrelativistic transition potential for the weak strangeness-changing reaction $ΛN \to NN$. The potential is based on a one meson exchange model (OME), where, in addition to the long-ranged pion, the exchange of the pseudoscalar $K, η$, as well as the vector $ρ, ω, K^*$ mesons is considered. Results obtained for different hypernuclear decay observables are compared to the available experimental data.
A combined analysis of new eta photoproduction data for total and differential cross sections, target asymmetry and photon asymmetry is presented. Using a few reasonable assumptions we perform the first model-independent analysis of the E0+, E2- and M2- eta photoproduction multipoles. Making use of the well-known A3/2 helicity amplitude of the D13(1520) state we extract its branching ratio to the eta-N channel, Gamma(eta,N)/Gamma = (0.08 +- 0.01)%. At higher energies, we show that the photon asymmetry is extremely sensitive to small multipoles that are excited by photons in the helicity 3/2 state. The new GRAAL photon asymmetry data at higher energy show a clear signal of the F15(1680) excitation which permits extracting an F15(1680)->eta,N branching ratio of (0.15 +0.35 -0.10)%.
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.
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 $p(γ,K^+)Λ$ are given.
The weak production of Lambdas in nucleon-nucleon scattering is studied in a meson-exchange framework. The weak transition operator for the $NN \to N Λ$ reaction is identical to a previously developed weak strangeness-changing transition potential $ΛN \to NN$ that describes the nonmesonic decay of hypernuclei. The initial $NN$ and final $YN$ state interaction has been included by using realistic baryon-baryon forces that describe the available elastic scattering data. The total and differential cross sections as well as the parity-violating asymmetry are studied for the reaction $pn \to pΛ$. These observables are found to be sensitive to the choice of the strong interaction potential and the structure of the weak transition potential.
With the advent of high precision proton accelerators, such as COSY in Jülich, or with medium energy accelerators like RCNP at Osaka, it may become possible to perform a direct study of the strangeness changing hadronic $np \to Λp$ process. In this work we study this reaction using a one-meson-exchange weak potential and realistic $NN$ and $YN$ strong interactions to distort both the initial $np$ and the final $Λp$ states. The calculated cross sections are very sensitive to the different mesons included in the weak transition, as well as to the strong $YN$ interaction model that generates the distorted final state.
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 $χ^2$ values. Moreover, predictions for the new Bonn SAPHIR data for $p(γ,K^+)Λ$ are given.
Kaon photoproduction on He3, gamma + He3 --> K+ + Hypertriton, is studied in the framework of the impulse approximation. Realistic He3 wave functions obtained as solutions of Faddeev equations with the Reid soft-core potential are used along with different Hypertriton wave functions. Results are compared for several elementary operator models, which can successfully describe the elementary kaon production off the proton up to a photon lab energy of k = 2.2 GeV. It is found that the corresponding cross sections are small, of the order of several nanobarns. It is also shown that the influence of Fermi motion is important, while the effect of different off-shell assumptions on the cross section is not too significant.
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, which generates the non-diffractive s- and u- channel resonance contributions. Additional t-channel $π^0$ and $σ$ exchanges are included for the $ω$ and $ρ^0$ production respectively to account for the large diffractive behavior in the small $t$ region as suggested by Friman and Soyeur. The numerical results are presented for the $ω$ and $ρ$ productions in four isospin channels with the same set of parameters, and they are in good agreement with the available data not only in $ω$ and $ρ^0$ productions but also in the charged $ρ$ productions where the additional t-channel $σ$ exchange does not contribute so that it provides an important test to this approach. The investigation is also extended to the $ϕ$ photoproduction, and the initial results show that the non-diffractive behavior of the $ϕ$ productions in the large $t$ region can be described by the s- and u- channel contributions with significantly smaller coupling constants, which is consistent with the findings in the similar studies in the QHD framework. The numerical investigation has also shown that polarization observables are essential for identifying so-called "missing resonances".
We report calculations for exclusive pion photoproduction on nuclei beyond the first resonance region in the distorted wave impulse approximation. The elementary operator contains contributions from the resonances $P_{33}(1232)$, $P_{11}(1440)$, $D_{13}(1520)$, $D_{33}(1740)$, $S_{11}(1535)$, $S_{11}(1650)$, and $F_{15}(1680)$, in addition to the usual Born plus vector meson contributions. It gives a good description of single pion photoproduction data up to about 1.1 GeV. Final state interactions are incorporated via optical potentials and are found to be substantial in the coincidence cross sections, but insensitive to the photon asymmetry. Clear signatures of possible medium modifications of the $D_{13}$ and $F_{15}$ resonances are predicted.
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 $χ^2$ values as compared to Ohta's prescription.
Violations of the $Δ$I=1/2 rule are investigated in the nonmesonic weak hypernuclear decay using a weak $Λ$N$\to$NN transition potential based on meson exchange. While the weak $Δ$I=3/2 matrix elements of baryons with pseudoscalar mesons are known to be very small, the same matrix elements of baryons with vector mesons, evaluated in the factorization approximation, are found to be significant. Within the uncertainties of the factorization approximation we find that the total rate increases by at most 6% lying within the error bars of the more recent experimental result. The neutron- to proton-induced rate, on the other hand, can change by up to a factor of two, while the asymmetry parameter is strongly affected as well.
An investigation of $γp\to ωp$ is presented in a constituent quark model approach. The sparse data in the large $t$ region where the resonances dominate is well described within the model, while the diffractive behavior in the small $t$ region requires an additional t-channel exchange. Taking into account the t-channel $π^0$ exchange, we find a good overall agreement with the available data with only 3 free parameters. Our study shows that the differential cross section is not sensitive to s-channel resonances, however, the polarization observables are demonstrated to be very sensitive. Thus, measuring polarization observables is the crucial part of the vector meson photoproduction program in the search for "missing" resonances.
An unified approach for vector meson photoproduction is presented in the constit quark model. The s- and u-channel resonance contributions are generated using an effective quark vector-meson Lagrangian. In addition, taking into account $π^0$ and $σ$ t-channel exchanges for diffractive production, the available total and differential cross section data for $ω$, $ρ^0$, $ρ^+$, and $ρ^-$ photoproduction can be well described with the same quark model parameter set. Our results clearly indicate that polarization observables are essential to identify so-called "missing" resonances.
We present a combined analysis of the new eta photoproduction data for total and differential cross sections, target asymmetry and photon asymmetry. Using basic assumptions, this allows a model-independent extraction of the $E_{2-}$ and $M_{2-}$ multipoles as well as resonance parameters of the $D_{13}(1520)$ state. At higher energy, we show that the photon asymmetry is extremely sensitive to small multipoles that are excited by photons in the helicity 3/2 state. These could be, e.g., the $F_{15}(1680)$, the $F_{17}(1990)$, or the $G_{17}(2190)$ resonances.
Charged pion photoproduction is proposed as a new method to study halo nuclei. In particular, it is demonstrated that the reaction 17O(gamma,pi^-)17F(1/2+, 0.495 MeV) is well-suited to investigate the well-known proton halo in 17F which is not amenable to radioactive beam experiments. The cross sections are shown to be very sensitive to the halo structure of the loosely bound proton.
The formation and excitation of hypernuclei through kaon photoproduction is reviewed. Basic features of the production process are emphasized. The possibility of extracting new information on hypernuclear structure and on the wave function of the bound $Λ$ is discussed. New results are presented for the quasifree production process $A(γ, K Λ)B$. Observables of this reaction are shown to be sensitive to the $Λ$-nucleus final state interaction.
Preliminary results of the analysis of the reaction p(gamma,K0)Sigma+ are presented. We show the first measurement of the differential cross section and much improved data for the total cross section than previous data. The data are compared with model predictions from different isobar and quark models that give a good description of p(gamma,K+)Lambda and p(gamma,K+)Sigma0 data in the same energy range. Results of ChPT describe the data adequately at threshold while isobar models that include hadronic form factors reproduce the data at intermediate energies.