Search for Higgs Boson decays to charm quarks with the ATLAS experiment and development of novel silicon pixel detectors

The coupling of the Higgs boson to the third generation of fermions and heavy vector bosons has been experimentally well-established at the Large Hadron Collider (LHC), and the measured properties agree well with Standard Model (SM) expectation. Evidence of the Higgs coupling to second-generation quarks is still elusive, and measurements of Higgs boson decays to $c$-quarks are the next most promising decay mode to probe the validity of the SM. This thesis describes a search for $H\to c\bar{c}$ decays in the $VH$ production mode, using a $pp$ collision dataset with an integrated luminosity of 139 $\mathrm{fb^{-1}}$ at a center-of-mass energy of 13 TeV, collected with the ATLAS detector. The analysis includes three different channels based on the decay of the vector boson and uses dedicated machine learning algorithms to identify $c$-quarks in the experiment. The observed (expected) upper limit for the $VH(H\to c\bar{c})$ process is $26~(31^{+12}_{-8})$ times the predicted Standard Model cross-section times branching fraction, at the 95\% confidence level, and a direct constraint on the Higgs-charm coupling modifier of $|\kappa_c| < 8.5$ is set. In a combination of this analysis with a measurement of the $VH(H\to b\bar{b})$ process an upper limit of $|\kappa_c / \kappa_b|< 4.5$ is determined. Significant analysis improvements and a larger dataset are needed to observe $VH(H\to c\bar{c})$ decays in the future. At the High-Luminosity LHC (HL-LHC), the ATLAS detector will collect at least 3000 $\mathrm{fb^{-1}}$ of integrated luminosity over ten years, which requires significant improvements to the detector to cope with increased radiation damage and higher data rates. This thesis discusses the characterisation of the ATLAS pixel detector readout chip for the inner detector upgrade (ITk) for HL-LHC. The radiation tolerance of the chip is tested in irradiations using X-rays and protons. All readout chip components, such as the digital logic and analogue front-end, are sufficiently radiation-tolerant to withstand the 1 Grad total ionising dose expected at HL-LHC. Finally, the thesis describes monolithic pixel detectors as a possible technology for future pixel detectors and summarises the characterisation of a monolithic detector prototype in an X-ray beam test.