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EPJ A - Hadrons and Nuclei
Hadrons and Nuclei


Eur. Phys. J. A 11, 209-216 (2001)

Aspects of near-threshold neutral pion photoproduction off protons

V. Bernard1, N. Kaiser2 and U.-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

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(Received: 25 May 2001 / Revised version: 20 June 2001 Communicated by A. Schäfer)

Abstract
We investigate near-threshold neutral pion photoproduction off protons to fourth order in heavy-baryon chiral perturbation theory in the light of the new data from MAMI. We show that the unitarity cusp at the secondary $\pi^+n$ threshold is in agreement with expectations from the final-state theorem. We also analyze the fourth-order corrections to the P-wave low-energy theorems and show that potentially large $\Delta$ isobar contributions are cancelled by sizeable pion loop effects. This solidifies the parameter-free third-order predictions, which are in good agreement with the data.

PACS
12.20.Ds - Specific calculations.
12.39.Fe - Chiral Lagrangians.
25.20.Lj - Photoproduction reactions.

© Società Italiana di Fisica, Springer-Verlag 2001

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