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$$\bar{t}$ pair production at the very high transverse momentum are presented. When a top quark is produced with very high momentum the decay products are collimated in the direction of the quark, and they can be clustered within a single large cone sizejet. The decay products of top quarks with transverse momentum larger than 400 GeV are likely having this boosted configuration. A definition of a particle-level top jet is developed which is independent of the partonic configuration. The measurements are performed with proton-proton collision data collected by the CMS experiment at the LHC, at the center of mass energy of s = 13TeV and they are presented differentially in the transverse momentum of the top jets and differentially in the azimuthal angle $\Delta$$\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+P YTHIA8.