Prototype development for a new readout of the CB/A2 setup and Analysis of the relative branching fraction of the eta´ → omega gamma decay

Abstract

The A2 collaboration at the electron accelerator MAMI in Mainz uses an energy-tagged photon beam to produce light mesons off the nucleon to study quantum chromodynamics, the underlying theory of the strong interaction, in the low-energy regime below 1 GeV. The A2 detector system mainly consists of the 4π calorimeter Crystal Ball (CB) and the TAPS calorimeter in forward direction, which are ideally suited to detect final state photons in the given energy range. In 2014, three dedicated beam-times for the production of η' mesons off unpolarized protons yielded a data sample of (5.12 ± 0.19) × 10^6 η' mesons within an incident photon energy range of E_γ = 1.42...1.58 GeV. This thesis consists of two parts: First, the prototype development to assess possible upgrades of the existing data acquisition system and, second, the analysis of the branching fraction of the decay η' → ωγ relative to the reference channel η' → γγ. To achieve the results, numerous improvements and innovations related to deployed hardware and software were developed and implemented. The prototype development for the CB/A2 experiment setup is based on the GSI TRB3 platform, primarily consisting of a multi-purpose 4 + 1 FPGA printed circuit board. In this thesis, a feature extraction firmware for an already existing sampling analog-to-digital converter extension board is developed and successfully tested at the CB calorimeter, showing a sufficiently precise time and energy measurement at the limit of the given analog signal quality. Additionally, the applicability of a charge-to-time conversion front-end is investigated for other components of the A2 detector system. The pseudoscalar-vector-gamma decay η' → ωγ serves as an input to effective field theories of the strong interaction, in particular concerning η-η'-mixing and the consistent inclusion of vector mesons. In this thesis, a new analysis framework has been developed providing advanced calibration and tuning methods including kinematic fitting for photo-production experiments. The final result BR(η' → ωγ) = (1.82 ± 0.19(stat)) × 10^{−2} is inconsistent with the current PDG world average at 4.3σ stat. Systematic studies for this deviation are discussed and future work is outlined.