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 studied in the trilepton channel. The data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$ are used, corresponding to an integrated luminosity of $36.1\,\text{fb}^{-1}$. Events containing three identified leptons (electron and/or muon) and two jets, one of which is identified as a $b$-quark jet are selected. The major backgrounds come from diboson, $t\bar{t}$ and $Z\text{\,+\,jets}$ production. A neural network is used to improve the background rejection and extract the signal. The resulting significance of the signal is $4.2\sigma$ in the data and the expected significance is $5.4\sigma$. The measured cross-section is $600 \pm 170\,(\text{stat.}) \pm 140\,(\text{syst.})\, \text{fb}$.