Phenomenology of $t$$\bar{t}$H Production with Top Quark Running Mass and the Differential Cross-Section Measurement of $t$$\bar{t}$+b-jets Production in the Dilepton Channel at $\sqrt{s}$ = 13 TeV with the CMS Experiment

A phenomenology study and a differential cross-section measurement are presented in this thesis. For the phenomenology study, the differential cross-sections of the top quark pair production in association with a Higgs boson ($t$$\bar{t}$H) are calculated with the top quark running mass in the $\overline{MS}$ renormalization scheme for the first time. The cross-sections and their scale uncertainties are calculated as a function of the kinematic variables of the top quarks, the Higgs boson, the top quark anti-quark system, and the invariant mass of the $t$$\bar{t}$H system. Subsequently, the results are compared to calculations in the pole mass scheme. Overall, the scale uncertainties in the calculations using the $\overline{MS}$ scheme are found to be slightly reduced compared to the pole mass scheme. The second study presented in this thesis is the total and differential cross-section measurements of the top quark pair production with additional jets originating from the hadronization of bottom quarks ($t$$\bar{t}$ + b-jets) in the dilepton decay channel of the $t$$\bar{t}$ system at the center-of-mass energy $\sqrt{s}$ = 13 TeV. The analysis is performed using data recorded by the CMS detector in the Run II period of the LHC during the years 2016, 2017, and 2018, corresponding to an integrated luminosity of 137.19 fb$^{−1}$ . The measurement of the total production cross-section of the $t$$\bar{t}$+b-jets process is the most precise to date and the value is found to be σ$t$$\bar{t}$+b-jets = 4.91 ± 0.08(stat) $_{-0.43}^{+0.47}$(sys) pb. The absolute and normalized differential cross-sections are measured as a function of transverse momentum, the absolute value of the pseudo-rapidity, the invariant mass, and the distance of the additional b-jets. For the first time, the differential distributions of the sub-leading additional b-jet are measured at $\sqrt{s}$ = 13 TeV. Additionally, two top quark definitions are used to define the additional b-jets at the truth level: a parton-based and a purely particle-based approach. The results are compared to the state-of-the-art Monte Carlo simulations of the $t$$\bar{t}$ + jets and $t$$\bar{t}$ + b-jets processes. Overall, all simulations are found to describe the measured normalized distributions similarly well. However, the absolute differential cross-sections are best described by the Powheg+OpenLoops+Pythia8 and the PowHel+Pythia8(5FNS) $t$$\bar{t}$ + b-jets simulations.