Measurement of multi-differential cross sections for the production of top quark pairs plus additional jets in pp collisions at $\sqrt{s}$ = 13 TeV

This thesis presents measurements of multi-differential top quark pair ($t$$\bar{t}$) production crosssections in $pp$ collisions at the LHC with a centre-of-mass energy of 13 TeV using events containing two opposite-sign leptons. The analyzed dataset was recorded with the CMS detector in the years 2016, 2017 and 2018, and corresponds to an integrated luminosity of 137 fb$^{-1}$. Precision double- and triple-differential cross sections are measured as a function of observables describing the kinematic properties of the top quark, top antiquark, and $t$$\bar{t}$ system, for both parton and particle level definitions of these objects. Furthermore, single- and double differential cross sections as functions of these observables are extracted in the presence of additional jets in the event. The set of results contains also the single differential cross section of the number of additional jets for different ranges of minimum $p$$_{T}$ of the jets. Studies are also presented as function of the kinematics of the leading and sub-leading extra jets and the combined $t$$\bar{t}$ plus leading extra jet system. The obtained unfolded cross sections are compared to MC predictions based on Next-to-Leading order QCD matched to parton showers, and also to different beyond-NLO theory predictions. A study is performed on the possibility of using the double differential measurement as function of the top quark transverse momentum and $t$$\bar{t}$ invariant mass as a two-dimensional reweighting of the $t$$\bar{t}$ signal simulations at generator level, to improve the simulation. The effects of the application of such correction on different top and $t$$\bar{t}$ system observables are analyzed.