Simultaneous measurements of $t\bar{t}$ production with additional heavy-flavor jets in the semileptonic channel with the CMS experiment

The production of top quark-antiquark pair ($t\bar{t}$) in association with additional particles ($X$), such as the Higgs boson, or heavy-flavor jets, plays an important role in experimental studies at the Large Hadron Collider (LHC) at CERN. These processes provide a sensitive probe of the Standard Model (SM) and are important for improving the modeling of backgrounds in Higgs boson measurements and searches for physics beyond the SM. This thesis presents a simultaneous measurement of $t\bar{t}$ production in association with heavy-flavor jets using proton collision data collected data at the CMS detector at the LHC during Run-II at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The targeted processes include $t\bar{t}$ production with additional bottom and charm jets, as well as associated production with a Higgs boson with decay $\text{H}\rightarrow\text{b}\bar{\text{b}}$ and a Z boson decay with $\text{Z}\rightarrow\text{b}\bar{\text{b}}$. These processes are studied in the semileptonic $t\bar{t}$ channel, requiring exactly one isolated charged lepton (electron or muon) in the final state. ... mehrThe complex task of simultaneously measuring the $t\bar{t}+X$ processes is addressed using advanced machine learning techniques such as Graph Neural Networks. Collision events are represented as graphs of reconstructed physics objects, enabling the model to learn correlations among objects via message passing and attention mechanisms. The production cross sections of the processes of interest are parameterized in terms of signal-strength parameters defined as $\mu=\sigma_\text{obs}/\sigma_\text{SM}$, where $\sigma_\text{SM}$ denotes the SM prediction. The signal strengths are extracted from a simultaneous binned maximum likelihood fit, yielding $\mu_{t\bar{t}B}^{obs} =1.09^{+0.39}_{-0.28}$, $\mu_{t\bar{t}C}^{obs} =0.31^{+0.19}_{-0.18}$, $\mu_{t\bar{t}H}^{obs} =1.99^{+0.65}_{-0.55}$, and $\mu_{t\bar{t}Z}^{obs} =2.90^{+1.18}_{-1.04}$. The measured value of $\mu_{t\bar{t}B}$ is consistent with the SM prediction within uncertainties. The observed $t\bar{t}C$ production rate is lower than predicted by the simulation, while the $t\bar{t}H$ and $t\bar{t}Z$ production rates are higher than the SM expectation.