Ph.D. Theses

With the discovery of the Higgs boson in 2012 at the Large Hadron Collider at CERN,the final elementary particle predicted by the Standard Model of particle physics wasdiscovered. Since then the study of the Standard Model has focussed on precision mea-surements of various parameters and processes of the theory in the hope of measuringsubtle differences between the theoretically predicted values and those observed in na-ture. [...]

Methanol is one of the most widely produced chemicals globally, valued for its importanceas both a chemical feedstock and a potential fuel in the transition toward a carbon-neutraleconomy. The industrial synthesis of methanol from syngas mixtures of CO, CO₂, and H₂is facilitated by the use of the Cu-ZnO-Al2O3 (CZA) catalyst. [...]

This cumulative dissertation addresses Cu/ZnO-based model catalysts for the het-erogeneous synthesis of methanol. The catalysts were prepared following a surfacescience approach by growing Cu nanoparticles (NPs) under ultra-high vacuum(UHV) conditions on basal and vicinal ZnO single crystals. [...]

Two key measurements and two phenomenological studies of top quark pair ($\mathrm{t\bar{t}}$) production with the CMS experiment at the CERN Large Hadron Collider are presented. They include the first measurement of the $\mathrm{t\bar{t}}$ cross section at the new energy frontier, a detailed analysis of the $\mathrm{t\bar{t}}$ threshold culminating in the first observation of $\mathrm{t\bar{t}}$ bound state effects, and searches for new physics in $\mathrm{t\bar{t}}$ final states, such as heavy scalars, pseudoscalars or Axion-Like Particles.The inclusive $\mathrm{t\bar{t}}$ production cross section $\sigma_{\mathrm{t\bar{t}}}$ is measured for the first time at the world-record energy of $\sqrt{s} = 13.6\,\mathrm{TeV}$, using $1.21\,\mathrm{fb}^{-1}$ of early LHC Run 3 data. [...]

Quantum computing is an emerging interdisciplinary research field that is gaining increasing relevance in the 21st century. Quantum computers leverage unique quantum properties such as entanglement, superposition, and quantum interference to efficiently solve certain classes of computationally demanding tasks, where classical methods face significant limitations.An example is particle track reconstruction in collider experiments. [...]

AbstractThis thesis both proposes and experimentally explores the opportunity to operate TransitionEdge Sensors (TES) as detectors for Dark Matter (DM)-scattering by simultaneouslyemploying them as target and sensor. By exploiting their microcalorimetric capabilitiesand sensitivity to low energy depositions, competitive limits can be set on low mass DMinteractions based on electron scattering. [...]

Fundamental insights into complex quantum phases of many-body systems can be ob-tained by the investigation of the underlying particle interactions. The variety of compet-ing interactions between particles involved determines their behaviour in these quantumphases. [...]

A measurement of partial branching fractions of charmless semileptonic beauty meson decaysusing an inclusively reconstructed hadronic system is presented. These decays are denoted asB → Xuℓν, where Xu represents a hadronic system containing a u quark produced in the un-derlying quark-level b → u transition. [...]

One of the major outstanding questions in astroparticle physics is the origin of ultra-high energy cosmic rays. Because charged cosmic rays are deflected by (inter)galactic magnetic fields, they do not point back to their sources, and photons are scattered by the low-energy background radiation that is present throughout the universe. [...]