Aspects of Jet Grooming in Theory and Experiment for Boosted Topologies in the Context of the ATLAS 1-lepton $X\to HH\to b\bar{b}WW^*$ Analysis

In order to address strongly collimated final states in particle detectors, it is necessary to utilize jet objects possessing high spatial resolution. This requirement can be fulfilled by incorporating tracking information in the definition of jet objects. Examples of jet objects that perform this function include Track-Assisted-Reclustered (TAR) jets and Unified Flow Object (UFO) jets. In this thesis, TAR jets and UFO jets are compared in semi-leptonic boosted H → W W ∗ decays on Monte Carlo (MC) truth level for the Higgs-Strahlung process Z∗ → ZH → bbW W ∗, generated by Sherpa, and for samples of the ATLAS 1-lepton X → HH → bbW W ∗ analysis. The jet objects are implemented in a Rivet analysis and validated. The performance of both jet objects in the discrimination of the Higgs-Strahlung process against QCD background is studied, showing miniscule differences in the separation power. The effect of clustering the lepton originating from the leptonically decaying W boson into the jet of the hadronically decaying W boson is observed, pointing out the grave deformation of observed distributions caused by an additional lepton in the jet, if no technique is applied to retroactively remove it. It is further shown that the jet objects are stable under systematic variations. After providing similar performance of the jet objects for the Higgs-Strahlung process, the Sherpa samples are compared to ATLAS X → HH → bbW W ∗ samples, which include a detector simulation. The findings for the Higgs-Strahlung process are confirmed for the ATLAS samples, suggesting the analysis strategy built around TAR jets also works for UFO jets. Furthermore, a new version of V +jets background samples is validated for the use in the X → HH(/SH) → bbW W (∗) analyses. Explorations of the MC modelling to data in a newly constructed Z+jets control region show that the background modelling would not benefit from a scaling of the Z+jets background.