Observation of the associated production of a top quark and a Z boson at 13 TeV with ATLAS

Single top-quark production in association with a $Z$ boson, where the $Z$ boson decays to a pair of charged leptons, is measured in the trilepton channel. The proton--proton collision data collected by the ATLAS experiment from 2015 to 2018 at a centre-of-mass energy of 13 TeV are used, corresponding to an integrated luminosity of 139 $\text{fb}^{-1}$. Events containing three isolated charged leptons (electrons or muons) and two or three jets, one of which is identified as containing a $b$-hadron, are selected. The main backgrounds are from $t\bar{t}Z$ and diboson production. Neural networks are used to improve the background rejection and extract the signal. The measured cross-section for $t\ell^{+}\ell^{-} q$ production, including non-resonant dilepton pairs with $m_{\ell^+\ell^-} > 30$ GeV, is 97 $\pm$ 13 (stat.) $\pm$ 7 (syst.) fb, consistent with the Standard Model prediction.