Simultaneous SMEFT interpretation of the signal and backgrounds in the $VH,H {\rightarrow} b\bar{b}$ analysis and ATLAS DSS during the start of LHC Run 3

The differential cross-section measurement of associated Higgs boson production decaying to a pair of $b$-quarks ($VH,H {\rightarrow} b\bar{b}$) is sensitive to deviations with respect to the Standard Model (SM) predictions that can be interpreted as the effect of new physics which may exist beyond the LHC energy scale. This thesis presents for the first time a SM effective field theory (SMEFT) interpretation of the measurement that consistently considers the effective modifications to both the $VH$ signal and two main backgrounds, diboson and $V$+heavy flavour jet production. Dimension-6 Wilson coefficients, to which the analysis was not sensitive before, are constrained significantly better by the novel combined interpretation. Furthermore, the thesis demonstrates that the new approach does not alter the constraints for coefficients to which the signal interpretation is particularly sensitive to. In addition, two examples of contributions to the ATLAS Detector Safety System during the recommissioning campaign for LHC Run 3 to record more collisions for unprecedented SM precision tests are described, namely the detector safety during the ATLAS beam pipe bake-out and the new control room ventilation panel.