Development of a likelihood-based top quark mass measurement using semileptonically decaying $t\bar{t}$+jet events with the full Run 2 ATLAS dataset

The top quark is the most massive elementary particle in the SM, which makes it play an important role in this theory and many of its extensions. This thesis presents the development status of a measurement of the top quark mass in the on-shell renormalization scheme, the top quark pole mass (m pole t ), using semileptonically decaying top quark pair events produced in association with at least one energetic jet (t¯t+jet). Additionally, the top quark mass parameter in the Monte Carlo simulation (mMC t ), is determined using semileptonically decaying top quark pair events produced in the lepton+4 jets channel, without additional jets (tt¯). The measurement is performed using the proton-proton collision data at 13 TeV collected by the ATLAS experiment at the LHC during Run 2 (2015-2018), corresponding to an integrated luminosity of 140 fb−1 . Candidate tt¯/tt¯+jet events are required to contain one electron or muon, a significant amount of missing transverse momentum, and four or more jets, from which at least one is identified as a jet originating from the hadronization of a bquark. To reconstruct the observables used in this analysis, ρs = 340 GeV/ √stt¯+jet with √stt¯+jet = mtt¯+jet and Mlb, a machine learning model implemented within the SPA-Net framework is used. The value of mMC t , and the corrected distribution of the parton level normalized cross section with respect to ρs are determined with a statistical model based on a profile likelihood fit. For the top quark pole mass, the methodology outlined in this thesis achieves an expected precision level of ∆m pole t (total) ≃ +0.88 −0.78 GeV, while the top quark Monte Carlo mass parameter in the simulation is expected to be determined with a precision of ∆mMC t (total) ≃ 0.41 GeV. At this precision, the top quark mass values extracted using this methodology would be positioned among the most precise determinations of these quantities to date.