Measurement of triple self-coupling of the Higgs boson in the four lepton decay channel with the CMS experiment

This doctoral dissertation presents the research performed in the area of detector technology development and of the measurement of the Higgs boson self-coupling using data from the CERN Large Hadron Collider (LHC). A study on Ultra-Fast Silicon Detectors was carried out to in view of future LHC timing-detector upgrades. In particular, the inter-pad region was studied in order to optimize the design regarding inter-pad width and fill factor. It was found that an inter-pad width of ∼ 25 μm is achievable for certain types of design. Further, a novel measurement using an elastomer material was performed to measure the leakage current and the guard ring current. Complementary measurements at different temperatures were carried out. Altogether, the trend of the expected voltage-current curves is reproduced well, although with a worse precision for sensors with a deeper implant. The Higgs boson fiducial cross section in the H → ZZ → 4l decay channel at 13.6 TeV was measured using data collected by the CMS detector during 2022 (early Run-3 data taking period). The measured fiducial cross section value is consistent with the value predicted by the Standard Model within uncertainties, thereby validating the analysis strategy applied throughout the thesis. The central analysis of this doctoral dissertation determines the expected sensitivity to the Higgsboson self-coupling modifier κλ of the H → ZZ → 4l decay channel in its off-shell region (i.e. far from the Higgs pole mass). The study employs the SMEFT theoretical framework to model Beyondthe- Standard-Model effects and categorizes events into untagged, VBF, and VH topologies to enhance signal-to-background discrimination. In addition, dedicated discriminants are based on the matrixelement probabilities derived from the theory calculation of the signal and background differential rates. The resulting likelihood scan in the combination of the three categories yields the following expected interval: − 16.5 < κλ < 19.6.