EPJ

2023 Impact factor 2.6
EPJ A - Hadrons and Nuclei
Hadrons and Nuclei
Eur. Phys. J. A 4, 259-275

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

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
(email: bernard@lpt1.u-strasbg.fr)
2 Technische Universität München, Physik Department T39, James-Franck-Straße, D-85747 Garching, Germany
(email: nkaiser@physik.tu-muenchen.de)
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

Abstract
We evaluate the threshold matrix-element for the reaction $pp \to pp\pi^0$ 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 ${\cal A} = (2.7 - i\,0.3)$ fm4 can be reproduced by contributions from tree level chiral (long-range) pion exchange and short-range effects related to vector meson exchanges, with $\omega$-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, $\vert\vec p\,\vert \simeq \sqrt {Mm_\pi}$, with M and $m_\pi$ the nucleon and the pion mass, respectively. We furthermore investigate the reaction $pp\to p n \pi^+$ near threshold within the same approach. We extract from the data the triplet threshold amplitude as ${\cal B}= (2.8 -i\,1.5)$ fm4. Its real part can be well understood from (relativistic) tree level meson-exchange diagrams. In addition, we investigate the process $pp \to pp \eta$ near threshold. We use a simple factorization ansatz for the $pp\eta$ final-state interaction and extract from the data the modulus of the threshold amplitude, $\vert{\cal C}\vert = 1.32\,$fm4. With $g_{\eta N}=5.3$, this value can be reproduced by (relativistic) tree level meson-exchange diagrams and $\eta$-rescattering, whose strength is fixed by the $\eta N$ scattering length. We also comment on the recent near threshold data for $\eta '$-production.

PACS
13.60.Le Meson production - 25.40.Ep Inelastic proton scattering

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