Measurement of the Cross Sections of $t\bar{t}$ Production in the Boosted Regime with the CMS Detector at $\sqrt{s}$ = 8 TeV

In this work the measurement of the $t\bar{t}$ production in ``boosted'' topologies, i.e. with high transverse momenta of the top quark, $p_{T}(t) > 400$ GeV, in $pp$ collisions at the LHC is presented. It is the first measurement of this kind in the dileptonic final state. By the end of 2012 the CMS detector recorded a data sample which contains of the order of 100 thousand $t\bar{t}$ pairs, which further decay dileptonically ($t\bar{t} \rightarrow W^{+}b\,W^{-}\bar{b} \rightarrow l^{+}\nu b\,l^{-}\bar{\nu}\bar{b}$). The dileptonic final state has a unique signature -- two leptons (electrons or/and muons), which are well reconstructed in the detector. The small branching ratio (low statistics) of the dileptonic decay channel of the $t\bar{t}$ system is compensated by an almost background free measurement of the $t\bar{t}$ production. In the presented analysis the kinematics of the $t\bar{t}$ final state is fully reconstructed. For this purpose six kinematic constrains are used to reconstruct the two undetected neutrinos in the $t\bar{t}$ final state. Two specific techniques are employed to account for the requirements of the boosted top quark analysis. Firstly, the spatial isolation of the leptons from hadronic jets in the events, which is important to reject fake leptons out of jets, was optimized by implementing an additional isolation criterion based on the relative transverse momentum of the electron to the jet. Secondly, to effectively suppress significant migrations from the true low $p_{T}(t)$ values to the boosted region, a kinematic restriction on the ratio of the invariant mass of the visible objects from the $t\bar{t}$ decay to the reconstructed $t\bar{t}$ mass is applied. The obtained cross section results in the high $p_{T}(t)$ region have a good precision, which is comparable to the one from measurements in the semi-leptonic channel, where hadronically decaying top quarks are reconstructed in the boosted regime as a single ``fat jet''. The measured production cross sections in the dileptonic and in the semi-leptonic decay channels are consistent with each other. The production dynamics of the boosted top quarks is elucidated in this work in more detail -- for this several differential production cross sections as a function of kinematic observables, such as the rapidity of the boosted top quark, are determined. Similar studies are performed in this work in the region of high invariant masses of the $t\bar{t}$ system. The comparison to Standard Model predictions based on four different QCD Monte Carlo simulation programs evidences no significant discrepancies. However, one can observe some trends in particular distributions, which indicate that the models are far from being perfect.