Searches for supersymmetric particles decaying to W and Higgs bosons in events with an isolated lepton with the ATLAS detector

The Standard Model (SM) of particle physics is a theory that provides a description of all known elementary particles and their interactions except gravitational interactions. Nevertheless, there are a few open issues which cannot be explained by the SM. Supersymmetry (SUSY) is a well motivated extension of the SM which solves some of its shortcomings and predicts the existence of new particles. This thesis presents two searches for supersym- metric particles in proton-proton collisions delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector in 2015 and 2016. The first search considers a simplified SUSY model in which squarks and gluinos are pair-produced by the strong interactions in LHC collisions. In- spired by the discovery of the Higgs boson in 2012, this model includes Higgs bosons in the decay chain as well as W and Z bosons. The final state conside- red consists of one lepton (electron or muon), jets and missing transverse momentum. The analysis uses three signal regions which are designed to target the wide range of kinematics expected from squark or gluino decays. The obtained sensitivity has been compared with a reinterpretation of a pub- lished analysis which targets a slightly different supersymmetric model. The second search explores a simplified SUSY model of electroweak pair production of charginos (χ˜i±) and neutralinos (χ˜0j) in which the lightest chargi- no (χ˜1±) and next-to-lightest neutralino (χ˜02) decay to W and Higgs bosons, respectively. A final state with one lepton (electron or muon), two b-jets from the Higgs boson decay, and missing transverse momentum is explored us- ing three statistically orthogonal signal regions. The data are found to be compatible with the SM predictions in all signal regions.