Measurements of the $\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$ Process with Neural Networks at the CMS Experiment - Current Status and Future Prospects

Abstract

This thesis presents a measurement of the production of a Standard Model Higgs boson in association with a top quark-antiquark pair ($\text{t}\bar{\text{t}}\text{H}$), where the Higgs boson in candidate $\text{t}\bar{\text{t}}\text{H}$ events is required to decay into a bottom quark-antiquark pair ($\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$). The results are based on data recorded at the CMS experiment in the years from 2016 to 2018, corresponding to an integrated luminosity of 137.5$\,\text{fb}^{-1}$. In particular, the semileptonic decay mode of the $\text{t}\bar{\text{t}}$ system is discussed. The strategy for the discrimination of signal and background events is based on the multiplicity of b-tagged jets and an event categorization performed by feed-forward neural networks. The compatibility of the statistical model used for the measurement and the observed data is evaluated using different statistical tests, which indicate good agreement. At the time of writing this thesis, final tests for the estimation of the signal strength for the $\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$ process with respect to the Standard Model prediction $\mu = \sigma^\text{obs}/\sigma^\text{SM}$ are performed. Therefore, the sensitivity expected within the scope of the Standard Model prediction is discussed, which corresponds to an expected significance of $3.3\,$standard deviations with respect to the background-only hypothesis in the semileptonic channel. Additionally, the impact of the semileptonic analysis on the results obtained in a combined parameter estimation with analyses targeting the other $\text{t}\bar{\text{t}}$ decay modes is discussed. Moreover, the results are reinterpreted in the scope of the Simplified Template Cross Section (STXS) measurement, where the cross sections in five regions of the transverse momentum of the Higgs boson are estimated simultaneously. The expected results from the STXS interpretation are used to conduct first feasibility studies of measurements of effects beyond the Standard Model as described in the Standard Model Effective Field Theory for the $\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$ process within the CMS experiment. Finally, the sensitivity for $\text{t}\bar{\text{t}}\text{H}(\text{b}\bar{\text{b}})$ measurements with future data sets is discussed, which is expected to reach approximately $7\,\%$ with the estimated amount of data collected by the end of the High Luminosity LHC