Møller polarimetry for the BGO-OD experiment and cross section measurement of the reaction γp → K⁺Λ at the extreme forward angles

The BGO-OD experiment, located at the electron accelerator ELSA at the University of Bonn, is designed to study nucleon and hyperon excitations via meson photoproduction using an energy tagged bremsstrahlung photon beam, with the emphasis on understanding the reaction dynamics. It consists of a central BGO calorimeter with a magnetic spectrometer in forward directions. The physics program includes the measurement of polarization observables, some of which can be accessed using polarized photon beams. Circularly polarized photon beams can be produced via bremsstrahlung from an amorphous radiator using longitudinally polarized electrons. The degree of polarization of the photon beam depends on the transferred momentum from the beam electron to the beam photon, and the degree of polarization of the electron beam. The polarization of the electron beam is measured by a Møller polarimeter. The setup and commissioning of this polarimeter is described in this thesis. Using a circularly polarized photon beam, several double polarization observables can be measured. For associated strangeness photoproduction, such as γp → K+Λ the self analyzing weak hyperon decay, allows the determination of the recoil polarization without requiring an additional recoil polarimeter. γp → K+Λ is therefore one of the photoproduction final states with the most polarization observable data available. Large contributions from t-channel exchange mechanisms however still leave it difficult to determine resonance, s-channel contributions in this channel. To understand the t-channel contribution, the forward angles are essential, but even in the most simplest observable, the cross section, the angular coverage is not complete. In addition the available data disagrees with each other. The BGO-OD experiment is ideally suited to resolve these issues. The magnetic forward spectrometer covers the forward angular range and the BGO calorimeter enabled significant background reduction. The differential cross section of γp → K+Λ at extreme forward angles, from Θc.m.K+ = 3° to 24°, is measured in this thesis. The achieved statistical and systematic uncertainties are 10% and 30% respectively. Within these errors the cross section fits to the model prediction of the Bonn Gatchina partial wave analysis. For the future, more data is available for analysis, which will reduce the statistical uncertainty significantly. By the ongoing improvement of the determination of the photon flux and by better modeling the real detector efficiencies in simulated data, the systematic uncertainties can be improved by a factor of two