A search is presented for massive spin-1 Z' resonances decaying to a top quark and a heavy vector-like top quark partner T. The search is based on a 2.6 inverse femtobarns sample of proton-proton collisions at 13 TeV collected with the CMS detector at the LHC. The analysis is optimized for final states in which the T quark decays to a W boson and a bottom quark. The focus is on all-jet final states in which both the W boson and the top quark decay into quarks that evolve into jets. The decay products of the top quark and of the W boson are assumed to be highly Lorentz-boosted and cannot be reconstructed as separate jets, but are instead reconstructed as merged, wide jets. Techniques for the identification of jet substructure and jet flavour are used to distinguish signal from background events. Several models for Z' bosons decaying to T quarks are excluded at 95% confidence level, with upper limits on the cross section ranging from 0.13 to 10 pb, depending on the chosen hypotheses. This is the first search for a neutral spin-1 heavy resonance decaying to a top quark and a vector-like T quark in the all-hadronic final state.

This thesis presents two studies in a search for new heavy resonances with 139 fb-1 of proton-proton collisions delivered by the Large Hadron Collider at √s = 13 TeV as measured by the ATLAS detector. In many Standard Model extensions, the top quark plays an important role in the dynamics of electroweak symmetry breaking, and so the studies presented here focus on heavy resonance searches in the top-antitop decay channel and their combination with searches for heavy resonances decaying into other final states. The first study detailed in this thesis is a search for new heavy resonances in a semileptonically decaying top-antitop final state where no significant excesses of data were observed with respect to the background prediction. That being so, exclusion limits at the 95% confidence level were placed on possible resonant masses for a Z′ in a leptophobic topcolor-assisted technicolor model and for a bulk Randall-Sundrum graviton at 4.2 TeV and 1.0 TeV, respectively. This top-antitop search was combined with searches in orthogonal final states performed by the ATLAS Collaboration from which limit contours were placed at the 95% confidence level in the space of couplings to Standard Model particles for the Heavy Vector Triplet model.

A search for the production of heavy resonances decaying into top quark-antiquark pairs is presented. The analysis is performed in the lepton+jets and fully hadronic channels using data collected in proton-proton collisions at $\sqrt{s}$ =13 TeV using the CMS detector at the LHC, corresponding to an integrated luminosity of 2.6 inverse femtobarns. The selection is optimized for massive resonances, where the top quarks have large Lorentz boosts. No evidence for resonant ttbar production is found in the data, and upper limits on the production cross section of heavy resonances are set. The exclusion limits for resonances with masses above 2 TeV are significantly improved compared to those of previous analyses at $\sqrt{s}$ = 8 TeV.

This unique volume contains the materials of the XXIXth International Workshop on High Energy Physics. The content of the volume is much wider than just high-energy physics and actually concerns all the most fundamental areas of modern physics research: high-energy physics proper, gravitation and cosmology. Presentations embrace both theory and experiment.