München 2006 – wissenschaftliches Programm

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HK: Physik der Hadronen und Kerne

HK 8: Elektromagnetische und Hadronische Sonden

HK 8.2: Vortrag

Montag, 20. März 2006, 16:45–17:00, F

Double pion photoproduction off the proton — •Fabien Zehr — Institut für Physik, Klingbergstr. 82, 4056 Basel, Switzerland

The reactions γ^→p → π⁰ π⁰ p and γ^→p → π⁺ π⁰ n have been measured from threshold up to the second resonance region with the combined Crystal Ball/TAPS detectors at the Mainz MAMI accelerator.
Chiral perturbation theory [1] 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 measurement of these cross sections at threshold provides important tests of ChPT. In the second resonance region, particular attention is given to the P₁₁(1440) resonance and its decay into p(π⁰ π⁰)_s−wave and Δ π⁰. Measurement with polarized beams provide asymmetry observables that are very sensitive to the internal mechanism of the reaction [2].
The data have been taken in 04/05 using a beam of tagged linearly and circularly polarized photons up to 820 MeV. 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.

[1] V. Bernard et al.: Nucl. Phys. A 580 (1994) 475-499

[2] L. Roca: Nucl. Phys. A 748 (2005) 192-205