# Measurement of polarisation observables using linearly polarised photons with the crystal ball at MAMI

Howdle, David A. (2012) Measurement of polarisation observables using linearly polarised photons with the crystal ball at MAMI. PhD thesis, University of Glasgow.

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In order to further study and expand the kinematic coverage of polarisation observables in pseudoscalar meson photoproduction, a measurement of polarisation observables has been performed at the MAMI facility in Mainz, Germany. The measurement used a beam of linearly polarised photons using the coherent bremsstrahlung method and the Glasgow Tagged Photon Spectrometer. The photon beam was incident on an $lH_{2}$ target in order to produce the meson photoproduction reaction $\gamma p \to \pi^{0} p$. This target was housed in the centre of the Crystal Ball detector which was used to detect the reaction products. A carbon polarimeter was used to measure the polarisation of the recoiling proton through secondary scattering. The polarisation observables for pseudoscalar meson photoproduction which were measured included: $\Sigma$, the modulation induced in the reaction products by the linearly polarised photon beam; $O_{x}$, the transfer of linear polarisation from the beam to the recoiling proton; and $T$, the polarisation inherent in the target proton. These measurements were performed over a wide kinematic range in both photon energy and polar angle in the centre of mass system, and were compared to three partial wave analyses, SAID, MAID and Bonn-Gatchina. The results contribute to the ongoing search for a complete understanding of the nucleon's excitation spectrum, and significantly enhance the world dataset for these polarisation observables.