Gießen 2007 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 11: Elektromagnetische und Hadronische Sonden

HK 11.4: Talk

Tuesday, March 13, 2007, 15:00–15:15, A

Double pion photoproduction off the proton — •Fabien Zehr for the A2 collaboration — Institut für Physik, Universität Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland

Double pion photoproduction is an important tool to study the nucleon stucture as it brings new informations about resonances that couple weakly to the π N channel. Especially interesting is the study of the P₁₁(1440) decay into p(π⁰ π⁰)_s−wave and Δ π⁰ as well as the D₁₃(1520) → ρ⁺ n process. At threshold, the precise measurement of the cross sections provides an important test of chiral perturbation theory. ChPT predicts that the contribution of pion loops to the double π⁰ cross section at threshold is dominant, which is a unique case among other pion photoproduction channels.
The reactions γ^→p → π⁰ π⁰ p and γ^→p → π⁺ π⁰ n have been measured from threshold up to the second resonance at MAMI accelerator in Mainz. A beam of tagged linearly and circularly polarized photons up to 820 MeV has been used. This allows for the determination of asymmetries observables, a good tool for this study as they are very sensitve to the internal mechanism of the reaction. Decay particles were detected using the 4π spectrometer made of the Crystal Ball in combination with TAPS as forward wall. Preliminary results which show a very large improvement of the statistical quality of the data in comparison to previous experiments will be discussed.
This work isupported by Swiss National fund.