Searches for top-antitop quark resonances in semileptonic final states with the CMS detector

This thesis presents the results of two searches for a top-antitop quark ($t\bar{t}$) resonance in semileptonic final states using data collected by the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC). Evidence of resonant $t\bar{t}$ production would represent a clear sign of new physics beyond the Standard Model of particle physics. The first search is based on the full data set recorded by the CMS experiment in proton-proton (pp) collisions at a center-of-mass energy of $\sqrt{s}=8~\text{TeV}$ during the LHC Run-1, for a total integrated luminosity of $19.7~\text{fb}^{-1}$. The second search considers $2.6~\text{fb}^{-1}$ of pp collisions data recorded by the CMS experiment in the first year of the LHC Run-2 (2015) at the higher center-of-mass energy of $\sqrt{s}=13~\text{TeV}$. Both analyses make use of state-of-the-art techniques for the identification of top quarks produced with large transverse momentum. This approach maximizes the sensitivity of the analyses for high-mass $X\rightarrow t\bar{t}$ resonances ($M_{X} > 1~\text{TeV}$), which are characterized by the production of an increasing fraction of top quarks in the boosted regime. The thesis also presents a set of studies on the use of the angular observables of the $t\bar{t}$ system and the treatment of interference effects in $X\rightarrow t\bar{t}$ searches. No evidence for resonant $t\bar{t}$ production is found in the data sets considered and the invariant mass spectrum of the reconstructed $t\bar{t}$ system is used to set upper limits on the production cross section of a $X\rightarrow t\bar{t}$ resonance in various new physics models.