Observation of the associated production of a top quark and a Z boson in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Single top-quark production in association with a $Z$ boson 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 $13\,\text{TeV}$ are used, corresponding to an integrated luminosity of $139\,\text{fb}^{-1}$. Events containing three isolated charged leptons (electrons and muons) and two or three jets, one of which is identified as containing a $b$-hadron, are selected. The main backgrounds are from $t\bar{t}Z$, diboson, $t\bar{t}$ and $Z$+jets production. A neural network is 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 $m_{\ell^{+}\ell^{-}} > 30 \text{GeV}$, is $98\,\pm 12\,\text{(stat.)}\,\pm 8\,\text{(syst.)}\,\text{fb}$, consistent with the Standard Model prediction