Cross Section measurements of $t\bar{t}$ pair production in the boosted regime in proton-proton collisions at $\sqrt{s}=13$ TeV with the CMS experiment at the Large Hadron Collider

In this thesis measurements of the $t\bar{t}$ pair production at the very high transverse momentum are presented. When a top quark is produced with very high momentum the decay products are collimated in the direction of the quark, and they can be clustered within a single large cone size jet. The decay products of top quarks with transverse momentum larger than $400$ GeV are likely having this boosted configuration. A definition of a particle-level top jet is developed which is independent of the partonic configuration. The measurements are performed with proton-proton collision data collected by the CMS experiment at the LHC, at the center of mass energy of $\sqrt{s}=13$ TeV and they are presented differentially in the transverse momentum of the top jets and differentially in the azimuthal angle $\Delta \phi$ between the two top-jets. The results are compared to theory predictions provided from different Monte Carlo event generators: i.e Powheg+Pythia8, Powheg+Herwigpp and aMC@NLO+Pythia8.