Ph.D. Theses

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. [...]

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. [...]

The synthesis of nanomaterials in solution often involves multiple intermediate steps from the precursor to the final product, as outlined in the non-classical nucleation theory. The first step in a one-pot reaction typically begins with the dissolution of the precursors in a solvent, often promoted by the coordination of metal ions with specific ligands. [...]

When in 2020 the virus for the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) hitthe world, the response of the scientific community was tremendous. Scientists used not only state-of-the-art methodology but also reacted by developing effective countermeasures. [...]

Properties of polymers are largely governed by their structure on the nanoscale. Therefore, precise control over polymer morphology enables novel design strategies for bottom-up nanofabrication and the tailored fabrication of emerging functional nanomaterials. [...]

Optically driven fundamental processes initiating ultrafast magnetization dynamics in 4f rare-earth metals have been studied for many years. Attempts were made to explain the various demagnetization timescales observed for different 4f metals, considering e.g [...]