Abstract

I analyze the new threshold data for neutral pion photoproduction off protons in the framework of heavy baryon chiral perturbation theory. I show that indeed large loop corrections as predicted by ChPT are needed to understand the S-wave multipole E 0+ at threshold. The low energy theorem for the P-wave combination P 1 is confirmed by the data. Furthermore all relevant low energy constants can be understood within the framework of resonance saturation. Previous inconsistencies in the description of this reaction close to threshold are resolved.

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