Analysis of Θproduction in K<Superscript>+</Superscript>- Xe collisions

The reaction K⁺XeK⁰pX is investigated in a meson-exchange model including rescattering of the secondary protons with the aim to analyze the evidence for the ⁺(1540)-resonance reported by the DIANA Collaboration. We confirm that the kinematical cuts introduced by the DIANA Collaboration efficiently suppress the background to the K⁺nK⁰p reaction which may contribute to the ⁺(1540) production. We find that these kinematical cuts do not produce a narrow structure in the K^0p effective-mass spectra near 1540 MeV. We study the effect of a narrow ⁺-resonance of both positive and negative parity in comparison with the DIANA data. We show that the K⁺XeK⁰pX calculations without ⁺ contribution as well as the results obtained with a ⁺ width of 1 MeV are in comparably good agreement with the DIANA results. More dedicated experiments are called for to establish this exotic baryon resonance.     

Effective field theory approach to nuclear matter

Effective field theory provides a systematic approach to hardon physics and few-nucleon systems. It allows one to determine the effective two-, three-, and more-nucleon interactions which are consistent with each other. We present a project to derive bulk properties of nuclei from the effective nucleonic interactions. The text was submitted by the authors in English.     

The reaction πN → ππN in a meson-exchange approach

A resonance model for two-pion production in the pion-nucleon reaction is developed that includes information obtained in the analysis of pion-nucleon scattering in a meson-exchange model. The baryonic resonances Δ(1232), N^*(1440), N^*(1520), N^*(1535), and N^*(1650) are included. The model reproduces the total cross-sections up to kinetic energies of the incident pion of 400MeV and obtains the shapes of the differential cross-sections in reasonable agreement with the data.     

Isospin violation,light quark masses,and all that

Isospin violation is driven through the light quark mass difference and electromagnetic effects.I review recent progress in extracting the light quark mass difference and tests of the chiral dynamics of Quantum Chromodynamics in various reactions involving light as well as heavy quarks.     

Model-independent extraction of two-photon effects in elastic electron–proton scattering

We address the discrepancy between the Rosenbluth and polarization transfer data for the electromagnetic form factors of the nucleon. Assuming that the effect of two-photon corrections on the polarization transfer data is negligible, we obtain a model-independent estimate of the two-photon correction Δ2γ${\Delta }^{2\gamma }$. We analyze the polarization transfer data and the cross section data separately using dispersion relations. A central value as well as an error estimate for Δ2γ${\Delta }^{2\gamma }$ is then obtained from a comparison of the two analyses. The resulting values for Δ2γ${\Delta }^{2\gamma }$ are in good agreement with direct calculations available in the literature.     

Final state interactions in the decays <Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis>→<Emphasis Type="Italic">VPP</Emphasis>

We investigate the interplay between crossed channel final state interactions and the constraints from two-particle unitarity for the reactions J/ψ→V π π and VK[`(K)]VK\bar{K} , where V is either ω or φ. Using a model where the parameters are largely constrained by other sources, we find that, although small, crossed channel final state interaction can influence the amplitudes considerably, in special areas of phase space. These results cast doubt on the inapplicability of unitarity constraints on production amplitudes as recently claimed in the literature.     

Novel approach to pion and eta production in proton-proton collisions near threshold

We evaluate the threshold matrix–element for the reaction pp→ppπ⁰ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S–wave pp final–state interaction. The experimental value for the threshold amplitude A = (2.7 −i0.3) fm⁴ can be reproduced by contributions from tree level chiral (long–range) pion exchange and short–range effects related to vector meson exchanges, with ω-exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN–system above the pion production threshold due to the large external momentum, |p|≃ (Mm _π)^−1/2, with M and m _π the nucleon and the pion mass, respectively. We furthermore investigate the reaction pp→pnπ⁺ near threshold within the same approach. We extract from the data the triplet threshold amplitude as B = (2.8 −i1.5) fm⁴. Its real part can be well understood from (relativistic) tree level meson–exchange diagrams. In addition, we investigate the process pp→ppη near threshold. We use a simple factorization ansatz for the ppη final–state interaction and extract from the data the modulus of the threshold amplitude, |C|= 1.32 fm⁴. With g _ηN= 5.3, this value can be reproduced by (relativistic) tree level meson–exchange diagrams and η–rescattering, whose strength is fixed by the ηN scattering length. We also comment on the recent near threshold data for η^′–production. Received: 27 November 1998