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 -i 0.3) 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, 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 - i 1.5) 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 gηN = 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 ή-production.