Search for high-mass resonances in final states with a boosted-dijet resonance in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the CMS detector

The Standard Model (SM) of particle physics provides the best description of the fundamental constituents of the universe and their interactions. It is a robust theory, tested by many experiments with a high level of precision. However, there are several reasons to believe that the SM is not a fundamental theory, since, for example, it does not explain the huge difference between the electroweak and Planck scale (which is related to the so called "hierarchy problem" of the SM) and the values of the measured fermion masses. This led to the conception of several theories Beyond the Standard Model (BSM), which often foresee the existence of new resonances that could be detected in experiments at particle colliders, such as the Large Hadron Collider (LHC) at CERN. This thesis reports the first LHC search for high-mass hadronic resonances that decay to a parton and a second Lorentz-boosted resonance, which in turn decays into a pair of partons. Such resonances are predicted, for example, by BSM theories that foresee the existence of extra spatial dimensions, providing a solution to the open questions mentioned above. The search is based on data collected with the CMS detector in proton-proton collisions produced at the LHC at $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. The boosted resonance is reconstructed as a single wide jet with substructure consistent with a two-body decay. The high-mass resonance is thus considered as a dijet system. The jet substructure information and the kinematic properties of resonance decays are exploited to disentangle the signal from the large quantum chromodynamics multijet background. The dijet mass spectrum is analyzed for the presence of new high-mass resonances, and is found to be consistent with the standard model background predictions. Results are interpreted in a warped extra dimension model where the high-mass resonance is a Kaluza-Klein gluon, the boosted resonance is a radion, and the final state partons are all gluons. Limits on the production cross section are set as a function of the Kaluza-Klein gluon and radion masses. These limits exclude at 95% confidence level models with Kaluza-Klein gluon masses in the range from 2.0 to 4.3 TeV and radion masses in the range from 0.20 to 0.74 TeV. By exploring a novel experimental signature, the observed limits on the Kaluza-Klein gluon mass are extended by up to about 1 TeV compared to previous searches.