Black, KevinSherman, Alexander David2020-02-052020-02-052019https://hdl.handle.net/2144/39269The production of an electroweak single top quark produced in association with a Z boson is studied using 139fb^-1 of proton-proton collision data collected by the ATLAS experiment at the LHC over the period from 2015 to 2018, at a center-of-mass energy of √s = 13 TeV. This search is performed in the three lepton channel in which the Z boson decays leptonically and top quark decays semi-leptonically. Events are analyzed from a final state that contains three high-pt electrons and/or muons, two or three jets (one identified as coming from a b-quark), and substantial missing transverse energy. Major irreducible backgrounds come from WZ diboson and ttV + ttH + tWZ, with reducible backgrounds from tt + X and Z + jets events. A gradient boosting algorithm is trained and applied in order to separate signal and background distributions, with a binned likelihood fit applied to the discriminant produced by the algorithm. From this fit the null-hypothesis significance and cross-section measurement are extracted. The observed (expected) significance is 9.0(9.1)σ with an extracted cross-section of σ_tZq = 770 ± 108 (statistical) ± 53 (systematic) fb. This cross-section is compatible with prediction and therefore this measurement clears the threshold to declare discovery of tZq production.en-USAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/PhysicsAssociated productionATLASLHCSingle top quarkTop quarkThesis/Dissertation2019-12-24