We investigate the structure of the scalar mesons f 0 (975) and a 0 (980) within realistic meson-exchange models of the ππ and πη interactions. Starting from a modified version of the Jülich model for ππ scattering we perform an analysis of the pole structure of the resulting scattering amplitude and find, in contrast to existing models, a somewhat large mass for the f 0 (975) (m f 0 = 1015 MeV, Γ f 0 = 30 MeV). It is shown that our model provides a description of J/ψ → φππ/φKK data comparable in quality with those of alternative models. Furthermore, the formalism developed for the ππ system is consistently extended to the πη interaction leading to a description of the a 0 (980) as a dynamically generated threshold effect (which is therefore neither a conventional qq state nor a KK bound state). Exploring the corresponding pole 1 position the a 0 (980) is found to be rather broad (m a 0 = 991 MeV, Γ a 0 = 202 MeV). The experimentally observed smaller width results from the influence of the nearby KK threshold on this pole.

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We address the problem of pion production in low-energy -nucleus collisions. For the production mechanism, we assume a simple model consisting of a coherent sum of single pion exchange and the excitationfollowed by the decay into two pions and a nucleon-of the N*(1440) resonance. The production amplitude is modified by the final state interaction between the pions calculated using the chirally improved Jülich meson exchange model including the polarization of the nuclear medium by the pions. The model reproduces well the experimentally observed A→A cross sections, especially the enhancement with increasing A of the ϩ Ϫ mass distribution in the threshold region. ͓S0556-2813͑99͒50203-3͔PACS number͑s͒: 25.80. Hp, 13.75.Gx, 13.75.Lb, 21.65.ϩf The past ten years have witnessed a considerable increase in our understanding of the interaction. The success of chiral perturbation theory ͓1,2͔ in describing the nearthreshold behavior of the amplitude is a clear success for the effective field theory approach to the problem. Unfortunately, at energies above a few hundred MeV, where effects of unitarity become important, chiral perturbation theory becomes unwieldy and other approaches must be used to obtain a good description of the free scattering. One such approach is the meson exchange model developed by the Jülich group ͓3͔, which gives an excellent quantitative description of and K scattering phases up to about 1.5 GeV total cm energy. Another is the inverse amplitude method-a variant of the K-matrix approach-of Oset et al. ͓4͔. Here we will consider the application of the former to the problem of pion production by pions on nuclei in order to investigate some of the predictions of the model for the behavior of the scattering amplitude in the presence of a nuclear medium. The latest version of this model, which we refer to as the chirally improved Jülich model ͓5͔, respects the constraints on the S-wave scattering length imposed by chiral symmetry, while maintaining the quality of fit to the free scattering data. We briefly summarize here the main features of the model and present some of the results of the model for interactions in nuclear matter. For details of the model, we direct the reader to Refs. ͓5,6͔.In the chirally improved Jülich model, the interaction is driven by the exchange of mesons plus contact interactions, as in the Weinberg Lagrangian ͓7͔. This Lagrangian is chirally symmetric in the massless pion limit. The Born approximation for this interaction is used as the potential in a three-dimensional scattering equation of the BlankenbeclerSugar form ͓8͔. The solution of the scattering equation destroys chiral symmetry through both the partial summation of diagrams and the use of form factors, which are needed to ensure convergence of the integral equation. However, the off-shell behavior of the potential is prescribed in such a way as to preserve the scattering length constraint required by chiral symmetry.The motivation to ''chirally improve'' the original Jülich model resulted from studies of the beha...

Comment on "Confirmation of the Sigma Meson"In a recent Letter [1], Törnqvist and Roos reported on a re-analysis of pp S-wave phase shifts using a coupled channel formalism in which the dynamics is totally determined by s-channel resonances. They found in their solution a broad 2332 0031-9007͞96͞77(11)͞2332(1)$10.00

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The excitation of the proton into undetected multiparticle states (double diffraction dissociation) is an important background to single diffractive deep-inelastic processes ep →We present estimates of the admixture of the double diffraction dissociation events in all diffractive events. We find that in the J/Ψ photoproduction, electroproduction of the ρ 0 at large Q 2 and diffraction dissociation of real and virtual photons into high mass states X the contamination of the double diffraction dissociation can be as large as ∼ 30%, thus affecting substantially the experimental tests of the pomeron exchange in deep inelastic scattering at HERA. We discuss a possibility of tagging the double diffraction dissociation by neutrons observed in the forward neutron calorimeter. We present evaluations of the spectra of neutrons and efficiency of neutron tagging based on the experimental data for diffractive processes in the proton-proton collisions.

We have performed accurate G-matrix calculations for the hypernucleus A 6 0 using the one-and twoboson-exchange Julich hyperon-nucleon potentials. A finite-nuclei Pauli operator defined in terms of oscillator wave functions is employed, and treated essentially exactly in solving the coupled A7V and XA (/-matrix equation. Numerical stability concerning the choice of the fc-space mesh points for discretization and the accuracy of a finite-boundary approximation are examined. Our calculated spectrum for A 6 0 is in good agreement with experiments.

A model calculation for the reactions pp → pΛK + and pp → NΣK near their thresholds is presented. It is argued that the experimentally observed strong suppression of Σ 0 production compared to Λ production at the same excess energy could be due to a destructive interference between the π and K exchange contributions in the reaction pp → pΣ 0 K + . Predictions for pp → pΣ + K 0 and pp → nΣ + K + are given.In a recent measurement of the reactions pp → pΛK + and pp → pΣ 0 K + near their thresholds it was found that the cross section for Σ 0 production is about a factor of 30 smaller than the one for Λ production [1]. We want to report on an exploratory investigation of the origin of this strong suppression of the near-threshold Σ 0 production [2]. In particular we want to examine a possible explanation that was suggested in Ref.[1], namely effects from the strong ΣN final state interaction (FSI) leading to a ΣN → ΛN conversion. We treat the associated strangeness production in the standard distorted wave Born approximation. We assume that the strangeness production process is governed by the π-and K exchange mechanisms. In order to have a solid basis for our study of possible conversion effects we employ a microscopic Y N interaction model developed by the Jülich group (specifically model A of Ref. [3]). This model is derived in the meson-exchange picture and takes into account the coupling between the ΛN and ΣN channels.The vertex parameters (coupling constants, form factors) appearing at the πNN and KNY vertices in the production diagrams are taken over from the Jülich Y N interaction. The elementary amplitudes T KN and T πN →KY are taken from corresponding microscopic models [4,5] that were developed by our group. However, for simplicity reasons we use the scattering length and on-shell threshold amplitudes, respectively, instead of the full (off-shell) KN and πN → KY t-matrices. The off-shell extrapolation of the amplitudes is done by multiplying those quantities with the same form factor that is used at the vertex where the exchanged meson is emitted. Only s-waves are considered.We do not take into account the initial state interaction (ISI) between the protons. Therefore we expect an overestimation of the cross sections by a factor of around 3 in our calculation [6]. But since the thresholds for the Λ and Σ 0 production are relatively