Searches for new high mass resonances in the diboson channel with semileptonic llqq final state in pp collisions at √s = 13 TeV with the ATLAS detector

Standard Model predictions have been verified with high precision by different experiments over the years after its formulation, but despite these predictions, there are many aspects that make this model not completely satisfactory, like the matter-antimatter asymmetry in the universe or the neutrino oscillations. All this suggests that the Standard Model represents only a good approx of a more general theory that incorporates also the gravitational interactions. Many theoretical models of “Beyond Standard Model Physics” have been developed and a major goal of the physics programme at the Large Hadron Collider is the search for new phenomena that may become visible in high-energy proton–proton collisions. One possible signature for new physics is the production of a heavy resonance with the subsequent decay into a pair of vector bosons (WW, WZ, ZZ). Several models of physics beyond the Standard Model predict such a signature: the Heavy Vector Triplet (HVT) phenomenological Lagrangian model, where a new heavy vector triplet (W', Z') is introduced with the new gauge bosons degenerate in mass; warped extra dimensions Randall–Sundrum (RS) models predict spin-2 Kaluza–Klein (KK) excitations of the graviton, and spin-0 Radions. Searches in final states in which one boson decays in leptons and the other one decays in hadrons (semileptonic analyses) are reported in this thesis work. The results refers to an integrated luminosity of 139 fb−1 recorded with the ATLAS detector in 2015-2018 at √s = 13 TeV at the Large Hadron Collider (full Run-2 statistics). The semileptonic searches lead to different research analyses, according to the number of leptons in the final state. The work described in this thesis refers mainly to the llqq final state