Search for resonances produced in association with or decaying to a Z boson at large transverse momentum with the ATLAS detector at the LHC

This dissertation presents a generic search for resonances performed with Z boson events decaying to ee and μμ at the transverse moment greater than 100 GeV using proton-proton collisions data collected with the ATLAS detector, corresponding to an integrated luminosity of 139 fb^-1 with a center-of-mass energy of 13 TeV. The search aims for resonances produced in association with a Z boson and thus appearing as a recoil (X) of the Z boson or be an intermediate state (Y) decaying to a Z boson and the Standard Model particles or another resonance X. The requirement of transverse moment helps suppress the Standard Model background and enhance the sensitivity of the search. Six independent event categories are defined to be sensitive to different resonances decaying in different final states. The observables used for the search are the invariant mass of visible objects in the recoil system and the full final state. The invariant mass distributions in all six event categories and their combined one are examined for local excesses above a data-driven estimate of the Standard Model background. Large mass range for mass values down to 200 GeV and up to around 6 TeV can be studied depending on the mass spectra and categories. No significant excess is observed in any mass spectrum and event category considered. Upper exclusion limits on cross-section times branching ratio times acceptance are derived at 95% confidence level in a model-independent way for Gaussian-shaped signals with three different relative width values of 3%, 5%, and 10%. (author)