Measurement of the G Double-Polarisation Observable in Positive Pion Photoproduction

Establishing the resonance spectrum of the nucleon with accuracy would provide important new information about the dynamics and degrees of freedom of its constituents. The spectrum and properties of nucleon resonances are a fundamental test of the emerging predictions from Lattice QCD calculations and will guide refinements to QCD-based phenomenological models. Pion photoproduction is an excellent tool to study the nucleon resonance spectrum, as this channel is expected to couple strongly to most resonances. The new generation of measurements for this reaction, of which the measurement presented in this thesis forms a crucial part, will provide a great improvement in the quality of available experimental data. For the photoproduction process in particular, the use of photon beams and targets with high degrees of polarisation, coupled with large acceptance particle detectors is essential for disentangling the spectrum of excited states. There are many nucleon resonances predicted by recent Lattice QCD calculations and by phenomenological nucleon models which are only observed inconsistently by different analyses of the same experimental data or which are not observed at all. It is of upmost importance to establish if this means that the resonances do not exist in nature, reflecting inappropriate degrees of freedom in the theoretical description of the nucleon or if the current experimental measurements have not been sensitive enough. As such, there is a current world effort at modern tagged photon facilities to measure the “complete set” of photoproduction observables necessary to fully constrain the partial wave analyses used to extract the experimental excitation spectrum from the data. This thesis will present the first detailed measurement to date of positive pion photoproduction in the 730-2300 MeV photon energy (1400-2280 MeV centre-ofmass energy) region with a linearly polarised photon beam and a longitudinally