Measurement of the Production Cross-Section of a Single Top Quark in Association with a Z Boson in Proton-Proton Collisions at 13 TeV with the ATLAS Detector

The production of a top quark in association with a Z boson is investigated. The proton-proton collision data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of √s = 13 TeV are used, corresponding to an integrated luminosity of 36.1 fb⁻¹. Events containing three identified leptons (electrons and/or muons) and two jets, one of which is identified as a b-quark jet are selected. The major backgrounds are diboson, tt̅ and Z + jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance is 4.2σ in the data and the expected significance is 5.4σ. The measured cross-section for tZq production is 600 ± 170(stat.) ± 140(syst.)fb.