Search for electroweak supersymmetry in final states with a W boson and a Z boson with the ATLAS detector

Supersymmetry is one of the most promising extensions of the Standard Model, addressing several of its theoretical limitations and providing a natural dark matter can- didate in the form of the lightest supersymmetric particle. Among the various supersym- metric production mechanisms, the electroweak production of charginos and neutralinos is of particular interest due to its comparatively clean experimental signatures. The supersymmetric process under investigation is pp → ˜χ±1 ˜χ02 → W Z ˜χ01 ˜χ01. This work targets final states comprising one or two charged leptons and at least one jet deriving from the decays of either the W or Z boson, through the analysis of Monte Carlo simulation data collected by the ATLAS detector at the Large Hadron Collider. The primary objective of this study is to isolate the supersymmetric signal from the Standard Model background across different final states in channels with either one or two charged leptons separately, assuming the SM branching ratios for the W and Z boson decays. The core of the analysis lies in the definition and systematic study of signal regions, constructed via dedicated selection cuts optimised to maximise the cor- responding statistical significance. Boosted and resolved jet topologies are considered to target the hadronically decaying bosons for both analysis channels. The two channels are complementary to one another given that the W boson decays leptonically and the Z boson decays hadronically in the 1ℓ and, vice-versa, the Z boson decays leptonically while the W boson decays hadronically in the 2ℓ. The adopted optimisation strategy is the “Cut-and-Count” method, a well-established approach in which signal and back- ground yields are compared within a signal-like phase space region after the application of a set of sequential kinematic selection cuts. Although the process considered has been the subject of prior investigations, this work aims to both consolidate previous analysis strategies, improve the performance of the boosted selection with state-of-the-art boson tagging algorithms, and assess the impact of incorporating reconstructed events from the Run 3 Dataset in terms of statistical significance. Furthermore, a boosted topology in the dilepton channel is studied for the first time alongside the resolved channel, extending the scope of the previous analysis and providing complementary sensitivity to the targeted supersymmetric signal.