V. Bernard1 - N. Kaiser2 - Ulf-G. Meißner3
1 Université Louis Pasteur, Laboratoire de Physique Théorique 3-5, rue de
l'Université, F-67084 Strasbourg, France
2 Technische Universität München, Physik Department T39, James-Franck-Straße, D-85747 Garching, Germany
3 Forschungszentrum Jülich, Institut für Kernphysik (Theorie), D-52425 Jülich, Germany (email: Ulf-G.Meissner@fz-juelich.de)
Received: 27 November 1998 Communicated by W. Weise
We evaluate the threshold matrix-element for the reaction 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 fm4 can be reproduced by contributions from tree level chiral (long-range) pion exchange and short-range effects related to vector meson exchanges, with -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, , with M and the nucleon and the pion mass, respectively. We furthermore investigate the reaction near threshold within the same approach. We extract from the data the triplet threshold amplitude as fm4. Its real part can be well understood from (relativistic) tree level meson-exchange diagrams. In addition, we investigate the process near threshold. We use a simple factorization ansatz for the final-state interaction and extract from the data the modulus of the threshold amplitude, fm4. With , this value can be reproduced by (relativistic) tree level meson-exchange diagrams and -rescattering, whose strength is fixed by the scattering length. We also comment on the recent near threshold data for -production.
13.60.Le Meson production - 25.40.Ep Inelastic proton scattering