Measurement of the production cross-section of a single top quark in association with a 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 $\sqrt{s}=13\phantom{\rule{0.2em}{0ex}}\text{TeV}$ are used, corresponding to an integrated luminosity of $36.1\phantom{\rule{0.2em}{0ex}}{\text{fb}}^{-1}$. 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, $t\overline{t}$ and $Z+\text{jets}$ 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 $600±170\phantom{\rule{0.2em}{0ex}}\text{(stat.)}±140\phantom{\rule{0.2em}{0ex}}\text{(syst.)}\phantom{\rule{0.2em}{0ex}}\text{fb}$.