Measurement of differential cross sections in $t\bar{t}$ and $t\bar{t}$+jets production in the lepton+jets decay mode in $pp$ collisions at $\sqrt{s}=13$ TeV using 140 fb$^{-1}$ of ATLAS data including the extraction of $\alpha_\text{S}$ and in situ calibration of jets

This thesis presents measurements of absolute and normalised differential cross sections for $t\bar{t}$ production in the $\ell$+jets channel in proton$-$proton collisions at a centre-of-mass energy of 13 TeV with 140 fb$^{-1}$ of integrated luminosity recorded by the ATLAS detector at the LHC. Taking these measurements as input, an extraction of the strong coupling at the $Z$-boson mass scale and a test of the running of this coupling with the scale are performed. In addition, a study of the in situ calibration of the jet energy scale for jets using photon-plus-jets events in proton$-$proton collisions is also included. The differential cross section is measured using the data sample including the full Run 2 of the LHC. The cross sections are measured as functions of twenty different observables at particle level. The measurements focus on jet observables for the first time with an inclusive $t\bar{t}$ selection in addition to two exclusive channels with at least one or two extra jets. For the first time, next-to-next-to-leading-order simulations are compared with differential cross sections measured by ATLAS at particle level. These measurements are useful for testing the different parton shower and hadronisation models used in Monte Carlo event generators, in addition to reducing the uncertainties in the gluon parton density functions for protons. Additionally, these measurements are sensitive to the mass of the top quarks and the strong coupling. Effective field theory fits focused on top Wilson coefficients can use the normalised cross sections as input for searches beyond the Standard Model. The strong coupling at the scale of the $Z$-boson mass is measured and a test of the running of this fundamental parameter of Quantum Chromodynamics is performed. These measurements use three different absolute differential cross sections as input. The measurement of the running constitutes a test of the underlying gauge symmetry of Quantum Chromodynamics along with the number of different active flavours in a wide range of the scale in a single experiment. The in situ jet energy scale is measured using photon-plus-jets events with a topology where the photon and the most energetic jet move in opposite directions in the transverse plane. The data sample used includes the full Run 2 data-taking period of the LHC. This measurement will be used to improve the jet calibration applied in ATLAS analyses at a centre-of-mass energy of 13.6 TeV during Run 3. The in situ jet energy scale correction are below 2% in most of the measured range of the photon transverse momentum. The uncertainty in these measurements is below 1% in most of the measured range of the photon transverse momentum.