Electromagnetic properties of strange baryons in a relativistic quark model

We present some of our results for the electromagnetic properties of excited Σ hyperons, computed within the framework of the Bonn constituent-quark model, which is based on the Bethe-Salpeter approach. The seven parameters entering the model are fitted against the best-known baryon masses. Accordingly, the results for the form factors and helicity amplitudes are genuine predictions. We compare with the scarce experimental data available and discuss the processes in which Σ ^*'s may play an important role.     

Three-body hadronic structure of low-lying 1/2+ Σ and Λ resonances⋆

We discuss the dynamical generation of some low-lying 1/2⁺ Σ 's and Λ 's in two-meson one-baryon systems. These systems have been constructed by adding a pion in the S -wave to the ˉN pair and its coupled channels, where the 1/2^- Λ(1405) -resonance gets dynamically generated. We solve Faddeev equations in the coupled-channel approach to calculate the T -matrix for these systems as a function of the total energy and the invariant mass of one of the meson-baryon pairs. This squared T -matrix shows peaks at the energies very close to the masses of the strangeness -1 , 1/2⁺ resonances listed in the particle data book.     

Phenomenology of the Λ/Σproduction ratio in pp collisions

We show that the recently measured asymmetry in helicity-angle spectra of the Λ-hyperons, produced in the reaction pp → K ⁺Λp reaction, and the energy dependence of the total pp → K ⁺Λp cross-section can be explained consistently by the same Λp final-state interaction. Assuming that there is no final-state interaction in the Σ⁰ p channel, as suggested by the available data for the reaction pp → K ⁺Σ⁰ p, we can also reproduce the energy dependence of the Λ/Σ⁰ production ratio and, in particular, the rather large ratio observed near the reaction thresholds. The nominal ratio of the Λ and Σ⁰ production amplitudes squared, i.e. when disregarding the final-state interaction, turns out to be about 3, which is in line with hyperon production data from proton and nuclear targets available at high energies.     

The radiative decay of the <Emphasis Type="Italic">Λ</Emphasis>(1405) and its two-pole structure

We evaluate theoretically the radiative decay widths into γΛ and γΣ ⁰ of the two poles of the Λ(1405) found in chiral unitary theories and we find quite different results for each of the two poles. We show that, depending on which reaction is used to measure the Λ(1405) radiative decays, one gives more weight to one or the other pole, resulting in quite different shapes in the γΛ(Σ ⁰) invariant-mass distributions. Our results for the high-energy pole agree with those of the empirical determination of the γΛ and γΣ ⁰ radiative widths (based on an isobar model fitting of the K^-p atom data), which are sometimes referred to as “experimental data”. We have made a detailed study of the K ^- p→π ⁰ γΛ(Σ ⁰) and π ^- p→K ⁰ γΛ(Σ ⁰) reactions and have shown that they, indeed, lead to different shapes for the γΛ(Σ ⁰) invariant-mass distributions.     

On polarization of strange baryons in reactions p + p↦p + Λ0 + K+ and p + p↦p + Σ0 + K+ near thresholds

Polarization properties of strange baryons produced in pp reactions, p + p↦p + Λ⁰ + K⁺ and p + p↦p + Σ⁰ + K⁺, near thresholds of the final states pΛ⁰K⁺ and pΣ⁰K⁺ are analysed relative to polarizations of colliding protons. The cross-sections for pp reactions are calculated within the effective Lagrangian approach accounting for strong pp rescattering in the initial state of colliding protons with a dominant contribution of the one-pion exchange and strong final-state interaction of daughter hadrons (Eur. Phys. J. A 9, 425 (2000)). Received: 22 October 2001 / Accepted: 14 November 2001     

Radial excitations of low–lying baryons and the Z + penta–quark

Abstact: Within an extended Skyrme soliton model for baryons the interplay between the collective radial motion and the SU(3)–flavor–rotations is investigated. The coupling between these modes is mediated by flavor symmetry breaking. Collective coordinates which describe the corresponding large amplitude fluctuations are introduced and treated canonically. When diagonalizing the resulting Hamiltonian flavor symmetry breaking is fully taken into consideration. As eigenstates not only the low–lying (1/2)⁺ and (3/2)⁺ baryons but also their radial excitations are obtained and compared to the empirical data. In particular the relevance of radial excitations for the penta–quark baryon Z ⁺ (Y=2, I=0, J ^π=(1/2)⁺) is discussed. In this approach its mass is predicted to be 1.58 GeV. Furthermore the widths for various hadronic decays are estimated which, for example, yields Γ(Z ⁺→NK) ∼ 100 MeV for the only permissible decay process of the Z ⁺. Received: 20 April 1998 / Revised version: 29 May 1998     

Strangeness production on the nucleon and on nuclei

The coupled-channel Giessen Model is used to investigate meson production on the nucleon simultaneously in reactions with hadrons and the photon. The essential aspects of the model are discussed. Results for strangeness producing KΛ reactions on the proton and eta photoproduction on the proton and the neutron, respectively, are presented. A resonance model is used to explore the production of hypernuclei in proton and photon induced reactions.     

String model description of polarisation and angular distributions in ¯pp → ¯ΛΛ at low energies

The observed polarisation of Λ hyperons from the inclusive pA → ΛX reaction at high energies has previously been well described within the Lund string model through polarised sˉs quark pair production in the string breaking hadronisation process. This model is here applied to the exclusive ˉpp → ˉΛΛ reaction at low energies and compared to available data sets down to an incident-beam momentum of 1.835 GeV/c. This required an extension of the diquark scattering model to involve three components: an isotropic part relevant close to the threshold, a spectator part and a forward-scattering part as in pA → ΛX at high energies. The observed angular distributions are then reproduced and, for momentum transfers above | t'| = 0.2 GeV², agreement with the measured polarisation is also obtained. Received: 2 April 2002 / Accepted: 24 July 2002 / Published online: 3 December 2002 RID="a" ID="a"Present address: Department of Neutron Research, Uppsala University, Box 525, S-75120 Uppsala, Sweden; e-mail: Stephan.Pomp@tsl.uu.se. RID="b" ID="b"Present address: ESRF, F-38043 Grenoble, France. Communicated by A. Sch?fer     

P-wave charmonium decays into baryon and antibaryon pairs in quark pair creation model

A simple quark pair creation model is introduced to study exclusive decays of _{{c_J}} into baryon-antibaryon pairs. With this simple model, some exclusive decay processes, for example, _{c0} B¯ (B = ,⁰,^-) are investigated and their decay widths are evaluated by inclusion of the properties of outgoing baryons, and the results show that the strengthened decay channels _{{c_J}} ¯(J = 0, 2) are easily understood by considering only the color singlet contribution of P-wave charmonium.     

Hyperon production at COSY-TOF

The strangeness production program at the COSY-TOF experiment is discussed. The apparatus is shown emphasizing the technique to measure delayed decays. Results obtained for the reactions pp→K ⁺ Λp and pp→K ⁰ Σ ⁺ p are discussed.