Determination of the η mass from the production threshold for the γp → pη reaction

This thesis is dedicated to a new precise determination of the η meson mass based on a measurement of the threshold for the γp → pη reaction. This experiment was performed in the years 2004/2005 using the Crystal Ball/TAPS detector setup and the recently developed tagger focal-plane microscope detector at the MAMI-B facility in the Institut für Kernphysik of the Johannes Gutenberg-Universität Mainz. The real photon beam was produced by Bremsstrahlung of the 883 MeV electrons from MAMI-B on a thin diamond radiator. The absolute electron energy of the incident beam was precisely determined in the 3rd race-track microtron of MAMI-B. The tagged photon energies were determined using the Glasgow photon tagging spectrometer (tagger). The tagger focal-plane microscope detector was used for the first time in this experiment. It was placed in front of the main focal-plane detector array and improved the tagged photon energy resolution, covering the energy region around the η production threshold ( E thr = 707 MeV) from E γ = 674 MeV to E γ = 730 MeV at an electron beam energy E 0 = 883 MeV. Made of 96 scintillator strips overlapping to one third, the tagger microscope provided 191 tagging channels with an energy resolution of about 290 keV per channel compared to approximately 2 MeV available from the main focal-plane detector. The liquid hydrogen target was located at the center of the Crystal Ball detector. The Crystal Ball, covering polar angles between θ = 20 ° and θ = 160 ° , consisted of 672 NaI crystals. In order to distinguish between neutral and charged particles detected by the Crystal Ball, the system was equipped with a particle identification detector (PID) made of plastic scintillator. The forward wall detector, TAPS, had 510 BaF 2 hexagonally shaped crystals, each equipped with a 5 mm thick plastic scintillator for identifying charged particles. The TAPS detector, intended for detecting particles in the forward direction ( θ = 4 ° -20 ° ), was located at a distance of 173 cm from the Crystal Ball center, ...