Mainz 2012 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 15: Hadronenstruktur und -spektroskopie

HK 15.8: Talk

Tuesday, March 20, 2012, 18:30–18:45, P 1

Measurement of the Beam Asymmetry in Neutral Pion Photoproduction from the Proton near Threshold — •David Hornidge — Department of Physics, Mount Allison University, Sackville, NB, Canada

A precise measurement of neutral pion photoproduction with a linearly polarized, tagged, photon beam and almost 4-pi detector in the threshold region (and higher) was performed at the Mainz Microtron. The Glasgow-Mainz tagged photon facility along with the Crystal Ball/TAPS detector system and a cryogenic liquid hydrogen target were used to obtain the total and differential cross sections simultaneously with the photon-beam asymmetry. This allowed for a precise determination of the S-wave amplitude, all three P-wave amplitudes and, for the first time, their energy dependence. The unitary cusp in the S-wave amplitude arising from charged pion re-scattering was also examined, leading to the extraction of the cusp function for the real part of the electric dipole amplitude. Results for the photon-beam asymmetry differ from the only previous measurement, which was done with the TAPS detector alone (PRL 87 232501). These data provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of convergence.