A search for heavy neutral gauge bosons and dark-sector particles with the ATLAS detector

The Standard Model of particle physics is currently the best theory we have for describing the most fundamental particles and interactions in Nature, and its validity is proven over a large range of energies. However, it is known that the Standard Model is an incomplete theory, motivating experimental searches for new fundamental particles and phenomena. This thesis presents a search for new physics in final states with two leptons ($e^+e^-$/$\mu^+\mu^-$) and missing transverse energy. The targeted beyond Standard Model signal scenario is realised through production of a new neutral gauge boson ($Z'$) in association with dark-sector particles. The search is carried out using the complete set of data from proton-proton collisions at $\sqrt{s}=13$ TeV recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. No significant excess above the expected Standard Model background is observed. Two benchmarks models are considered in the search, referred to as the light-vector model and the dark-Higgs model. The benchmark models are used to optimise the search sensitivity, and a set of signal regions are defined as three bins in the $\text{E}_{\text{T}}^{\text{miss,sig}}$ variable. Limitations of the search, for example related to background statistics and statistical uncertainties, are taken into account when designing the signal regions. The results of the search are obtained by scanning across the dilepton invariant mass spectrum in the signal regions. Exclusion limits on the production cross-sections of the benchmark models, and on the coupling between the $Z'$ and the dilepton system, are presented for $Z'$ masses between $200$ GeV and $1$ TeV. The backgrounds from Standard Model processes are mainly estimated using Monte Carlo simulated data, with exception of the backgrounds from fake and non-prompt leptons which are estimated with a data-driven method.