🎓 PhD Research Project : Measurement of the ttH(bb) process in the dilepton final state with machine learning techniques at the CMS experiment

🎓 Thesis: Link to Thesis

📊 Presentation Slides: Link to Presentation

This thesis marks a significant advancement in the field of particle physics, presenting the first-ever measurement of the associated production of a Higgs boson and a top-quark-antiquark pair (ttH) in final states with two leptons, where the Higgs boson decays into a bottom-quark-antiquark pair (H->bb). The research utilizes proton-proton collision data collected at the CERN LHC with the CMS experiment at a center-of-mass energy of 13 TeV during 2016-2018 of Run 2, corresponding to an integrated luminosity of 138 fb-1.

In the Standard Model (SM), the Higgs boson interacts with elementary particles through a Yukawa-type interaction, with a coupling strength proportional to the particle mass. As the top quark is the heaviest elementary particle known to date, the ttH process is a crucial probe of the top-Higgs Yukawa coupling, providing vital insights into the SM nature of the Higgs boson. Moreover, the Higgs boson's H->bb decay presents the largest branching fraction in the SM, making it experimentally attractive as a final state.

The primary challenge in this study lies in the dominant background contributions arising from tt events containing additional jets not originating from the tt system (tt + jets), with tt + bb processes forming an almost irreducible background to the ttH, H->bb signal. To enhance the sensitivity of the measurement, innovative multivariate analysis techniques, specifically Artificial Neural Networks, are employed to extract the signal.

This research conducts comprehensive assessments of the performance and optimization of discriminants trained using Artificial Neural Networks alongside improved modeling of the dominant background contributions. The results are interpreted in terms of the inclusive ttH signal-strength modifier (μttH) and the first ttH, H->bb STXS measurement, serving as a powerful tool to test the SM predictions and explore new physics.

The statistical model used for measurements effectively describes the observed data, with the observed best-fit value for μ being 0.34+0.17(stat.)+0.20(syst.) for the combination of all decay channels. Additionally, the thesis delves into the luminosity measurement for the CMS experiment, calibrated using the Van der Meer method. Notably, an improved method is introduced to estimate the bias resulting from the assumption of factorizable proton densities of the beams, reducing the systematic uncertainty for measuring the integrated luminosity.

This research provides valuable insights into the interactions between Higgs boson and top quarks, further confirming the validity of the Standard Model while laying the groundwork for future discoveries in particle physics. The adoption of machine learning techniques enhances data analysis capabilities, offering exciting prospects for continued research and exploration at the forefront of particle physics.